PCI Section Rework

1) Minor changes from must to MUST etc.
2) More references using \ref.
3) Move section on capabilities first, before we talk about the common
   config layout.  The previous order made sense for legacy.
4) Make explicit subsections for each type of capability and move more
   information into them.
5) Make it clear that there must be one or more.
6) Include 'struct virtio_pci_cap cap;' in struct virtio_pci_cfg_cap to
   match virtio_pci_notify_cap.
7) Explicitly note there's no way to negotiate the queue size for a
   legacy device.
8) Fix old language on config change event: config is not in the pci
   configuration space.
9) Explicitly state what the driver should do to use virtio_pci_cfg_cap.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio@213 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652

1 files changed, 221 insertions, 213 deletions

diff --git a/content.tex b/content.tex
index 37850c8..9713817 100644
--- a/content.tex
+++ b/content.tex
@@ -809,35 +809,138 @@ All drivers MUST match devices with any Revision ID, this
 is to allow devices to be versioned without breaking drivers.
 
 \subsubsection{Legacy Interfaces: A Note on PCI Device Discovery}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Discovery / Legacy Interfaces: A Note on PCI Device Discovery}
-Transitional devices must have a Revision ID of 0 to match
+Transitional devices MUST have a Revision ID of 0 to match
 legacy drivers.
 
-Non-transitional devices must have a Revision ID of 1 or higher.
+Non-transitional devices MUST have a Revision ID of 1 or higher.
 
-Both transitional and non-transitional drivers must match
+Both transitional and non-transitional drivers MUST match
 any Revision ID value.
 
 \subsection{PCI Device Layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout}
 
 The device is configured via I/O and/or memory regions (though see
-VIRTIO_PCI_CAP_PCI_CFG for access via the PCI configuration space).
+\ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / PCI configuration access capability} for access via the PCI configuration space).
 
-These regions contain the virtio header registers, the notification register, the
-ISR status register and device specific registers, as specified by Virtio
-Structure PCI Capabilities.
+There may be different widths of accesses to the I/O region; the driver
+MUST access each field using the “natural” access method (i.e. 32-bit accesses for 32-bit fields, etc).  All multi-byte fields are little-endian.
 
-There may be different widths of accesses to the I/O region; the
-“natural” access method for each field must be
-used (i.e. 32-bit accesses for 32-bit fields, etc).
+\subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / Virtio Structure PCI Capabilities}
+
+The virtio device configuration layout includes a common configuration header, notification area, ISR status area and a device-specific configuration area.
+
+Each structure can be mapped by a Base Address register (BAR) belonging to
+the function, or accessed via the special VIRTIO_PCI_CAP_PCI_CFG field in the PCI configuration space.
+
+The location of each structure is specified using a vendor-specific PCI capability located
+on the capability list in PCI configuration space of the device.
+This virtio structure capability uses little-endian format; all fields are
+read-only unless stated otherwise:
+
+\begin{lstlisting}
+struct virtio_pci_cap {
+        u8 cap_vndr;    /* Generic PCI field: PCI_CAP_ID_VNDR */
+        u8 cap_next;    /* Generic PCI field: next ptr. */
+        u8 cap_len;     /* Generic PCI field: capability length */
+        u8 cfg_type;    /* Identifies the structure. */
+        u8 bar;         /* Where to find it. */
+        u8 padding[3];  /* Pad to full dword. */
+        le32 offset;    /* Offset within bar. */
+        le32 length;    /* Length of the structure, in bytes. */
+};
+\end{lstlisting}
+
+This structure can be followed by extra data, depending on
+cfg_type, as documented below.  The device MAY append extra data
+or padding to any structure beyond that, the device MUST accept a cap_len field
+which is larger than specified here.
+
+The fields are interpreted as follows:
+
+\begin{description}
+\item[cap_vndr]
+        0x09; Identifies a vendor-specific capability.
+
+\item[cap_next]
+        Link to next capability in the capability list in the configuration space.
+
+\item[cap_len]
+        Length of this capability structure, including the whole of
+        struct virtio_pci_cap, and extra data if any.
+        This length MAY include padding, or fields unused by the driver.
+
+\item[cfg_type]
+        identifies the structure, according to the following table:
+
+\begin{lstlisting}
+/* Common configuration */
+#define VIRTIO_PCI_CAP_COMMON_CFG        1
+/* Notifications */
+#define VIRTIO_PCI_CAP_NOTIFY_CFG        2
+/* ISR Status */
+#define VIRTIO_PCI_CAP_ISR_CFG           3
+/* Device specific configuration */
+#define VIRTIO_PCI_CAP_DEVICE_CFG        4
+/* PCI configuration access */
+#define VIRTIO_PCI_CAP_PCI_CFG           5
+\end{lstlisting}
 
-PCI Device Configuration Layout includes the common configuration,
-ISR, notification and device specific configuration
-structures.
+        Any other value - reserved for future use. Drivers MUST
+        ignore any vendor-specific capability structure which has
+        a reserved cfg_type value.
 
-All multi-byte fields are little-endian.
+        The device MAY offer more than one structure of any type - this makes it
+        possible for the device to expose multiple interfaces to drivers.  The order of
+        the capabilities in the capability list specifies the order of preference
+        suggested by the device; drivers SHOULD use the first interface that they can
+        support.  For example, on some hypervisors, notifications using IO accesses are
+        faster than memory accesses. In this case, the device would expose two
+        capabilities with cfg_type set to VIRTIO_PCI_CAP_NOTIFY_CFG:
+        the first one addressing an I/O BAR, the second one addressing a memory BAR.
+        In this example, the driver SHOULD use the I/O BAR if I/O resources are available, and fall back on
+        memory BAR when I/O resources are unavailable.
+
+        Each structure is detailed individually below.
+
+\item[bar]
+        values 0x0 to 0x5 specify a Base Address register (BAR) belonging to
+        the function located beginning at 10h in Configuration Space
+        and used to map the structure into Memory or I/O Space.
+        The BAR is permitted to be either 32-bit or 64-bit, it can map Memory Space
+        or I/O Space.
+
+        Any other value is reserved for future use. Drivers MUST
+        ignore any vendor-specific capability structure which has
+        a reserved bar value.
+
+\item[offset]
+        indicates where the structure begins relative to the base address associated
+        with the BAR.
+
+\item[length]
+        indicates the length of the structure.
+
+        length MAY include padding, or fields unused by the driver, or
+        future extensions.
+
+        Drivers SHOULD only map part of configuration structure
+        large enough for device operation.  Drivers MUST handle
+        unexpectedly large length fields, but MAY check that length
+        is large enough for device operation.
+
+        For example, a future device might present a large structure size of several
+        MBytes.
+        As current devices never utilize structures larger than 4KBytes in size,
+        driver can limit the mapped structure size to e.g.
+        4KBytes to allow forward compatibility with such devices without loss of
+        functionality and without wasting resources.
+\end{description}
 
 \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Common configuration structure layout}
-Common configuration structure layout is documented below:
+
+The common configuration structure is found at the bar and offset within the VIRTIO_PCI_CAP_COMMON_CFG capability; its layout is below.
+
+The device MUST present at least one common configuration capability.
 
 \begin{lstlisting}
 struct virtio_pci_common_cfg {
@@ -865,19 +968,20 @@ struct virtio_pci_common_cfg {
 
 \begin{description}
 \item[device_feature_select]
-        The driver uses this to select which Feature Bits the device_feature field shows.
+        The driver uses this to select which feature bits the device_feature field shows.
         Value 0x0 selects Feature Bits 0 to 31, 0x1 selects Feature Bits 32 to 63.
-        The device MUST present 0 on device_feature for any other value.
+        The device MUST present 0 on device_feature for any other value, but the driver MUST NOT rely on this.
 
 \item[device_feature]
-        The device uses this to report Feature Bits to the driver.
-        Device Feature Bits selected by device_feature_select.
+        The device uses this to report which feature bits it is
+        offering to the driver: the driver writes to
+        device_feature_select to select which are presented.
 
 \item[driver_feature_select]
-        The driver uses this to select which Feature Bits the driver_feature field shows.
+        The driver uses this to select which feature bits the driver_feature field shows.
         Value 0x0 selects Feature Bits 0 to 31, 0x1 selects Feature Bits 32 to 63.
-        When set to any other value, reads from driver_feature
-        return 0, writing 0 into driver_feature has no effect.  The driver
+        When set to any other value, the device MUST return 0 on reads from driver_feature
+        return 0, and ignore writing of 0 into driver_feature.  The driver
         MUST not write any other value into driver_feature (a corollary of
         the rule that the driver can only write a subset of device features).
 
@@ -924,7 +1028,7 @@ struct virtio_pci_common_cfg {
 \item[queue_notify_off]
         The driver reads this to calculate the offset from start of Notification structure at
         which this virtqueue is located.
-        Note: this is *not* an offset in bytes. See notify_off_multiplier below.
+        Note: this is *not* an offset in bytes. See \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability} below.
 
 \item[queue_desc]
         The driver writes the physical address of Descriptor Table here.
@@ -936,18 +1040,87 @@ struct virtio_pci_common_cfg {
         The driver writes the physical address of Used Ring here.
 \end{description}
 
-\subsubsection{ISR status structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / ISR status structure layout}
-ISR status structure includes a single 8-bit ISR status field.
+\subsubsection{Notification structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability}
+
+The device MUST present at least one notification capability.
 
-\subsubsection{Notification structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification structure layout}
-Notification structure is always a multiple of 2 bytes in size.
-It includes 2-byte Queue Notify fields for each virtqueue of
-the device. Note that multiple virtqueues can use the same
-Queue Notify field, if necessary: see notify_off_multiplier below.
+The notification location is found using the VIRTIO_PCI_CAP_NOTIFY_CFG
+capability.  This capability is immediately followed by an additional
+field, like so:
+
+\begin{lstlisting}
+struct virtio_pci_notify_cap {
+        struct virtio_pci_cap cap;
+        le32 notify_off_multiplier; /* Multiplier for queue_notify_off. */
+};
+\end{lstlisting}
+
+The device MUST present an even cap.length of at least 2.
+
+The device MUST present notify_off_multiplier as an even power of 2,
+or 0.  The device MUST ignore a capability with notify_off_multiplier
+of 1.
+
+notify_off_multiplier field is combined with the queue_notify_off to
+derive the Queue Notify address within a BAR for a specific queue:
+
+\begin{lstlisting}
+        cap.offset + queue_notify_off * notify_off_multiplier
+\end{lstlisting}
+
+The BAR, offset and notify_off_multiplier are taken from the
+notification capability structure above, and the queue_notify_off is
+taken from the common configuration structure.
+
+For example, if notifier_off_multiplier is 0, all queues will use the same 
+Queue Notify address.
+
+\subsubsection{ISR status capability}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / ISR status capability}
+
+The device MUST present at least one VIRTIO_PCI_CAP_ISR_CFG capability.  This
+refers to at least a single byte, which contains the 8-bit ISR status field.
 
 \subsubsection{Device specific structure}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Device specific structure}
 
-Device specific structure is optional.
+The device MAY present at least one VIRTIO_PCI_CAP_DEVICE_CFG capability (some
+devices may not have any device specific structure).
+
+\subsubsection{PCI configuration access capability}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / PCI configuration access capability}
+
+The device MUST present at least one VIRTIO_PCI_CAP_PCI_CFG.  This
+creates an alternative (and likely suboptimal) access method to the
+common configuration, notification, ISR and device-specific regions.
+
+The capability is immediately followed by an additional field like so:
+
+\begin{lstlisting}
+struct virtio_pci_cfg_cap {
+        struct virtio_pci_cap cap;
+        u8 pci_cfg_data[4]; /* Data for BAR access. */
+};
+\end{lstlisting}
+
+To access a device region, the driver writes into the capability
+structure (ie. within the PCI configuration space) as follows:
+
+\begin{itemize}
+\item The driver sets the BAR to access by writing to the cap.bar field.
+
+\item The driver sets the size of the access by writing 1, 2 or 4 to
+  the cap.length field.
+
+\item The driver sets the offset within the BAR by writing to the
+  cap.offset field.  The driver MUST NOT write an offset which is not
+  a multiple of cap.length (ie. all accesses must be aligned).
+\end{itemize}
+
+At that point, the pci_cfg_data field will provide a window of size
+cap.length into the given cap.bar at offset cap.offset: writes will
+have the same effect as writes into the BAR, and reads will have the
+same effect and return the same value as reads from the BAR.
+
+The driver MUST perform reads/writes from/to pci_cfg_data of the same
+width as given by cap.length.
 
 \subsubsection{Legacy Interfaces: A Note on PCI Device Layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Legacy Interfaces: A Note on PCI Device Layout}
 
@@ -1032,179 +1205,13 @@ device.
 
 \paragraph{Virtio Device Configuration Layout Detection}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Device Initialization / Virtio Device Configuration Layout Detection}
 
-As a prerequisite to device initialization, driver executes a
-PCI capability list scan, detecting virtio configuration layout using Virtio
+As a prerequisite to device initialization, the driver scans the
+PCI capability list, detecting virtio configuration layout using the Virtio
 Structure PCI capabilities.
 
-Virtio Device Configuration Layout includes virtio configuration header, Notification
-and ISR Status and device configuration structures.
-Each structure can be mapped by a Base Address register (BAR) belonging to
-the function, located beginning at 10h in Configuration Space,
-or accessed though PCI configuration space.
-
-Actual location of each structure is specified using vendor-specific PCI capability located
-on capability list in PCI configuration space of the device.
-This virtio structure capability uses little-endian format; all bits are
-read-only:
-
-\begin{lstlisting}
-struct virtio_pci_cap {
-        u8 cap_vndr;    /* Generic PCI field: PCI_CAP_ID_VNDR */
-        u8 cap_next;    /* Generic PCI field: next ptr. */
-        u8 cap_len;     /* Generic PCI field: capability length */
-        u8 cfg_type;    /* Identifies the structure. */
-        u8 bar;         /* Where to find it. */
-        u8 padding[3];  /* Pad to full dword. */
-        le32 offset;    /* Offset within bar. */
-        le32 length;    /* Length of the structure, in bytes. */
-};
-\end{lstlisting}
-
-This structure can optionally be followed by extra data, depending on
-other fields, as documented below.
-
-Note that future versions of this specification will likely
-extend devices by adding extra fields at the tail end of some structures.
-
-To allow forward compatibility with such extensions, drivers must
-not limit structure size.  Instead, drivers should only
-check that structures are *large enough* to contain the fields
-required for device operation.
-
-For example, if the specification states 'structure includes a
-single 8-bit field' drivers should understand this to mean that
-the structure can also include an arbitrary amount of tail padding,
-and accept any structure size equal to or greater than the
-specified 8-bit size.
-
-The fields are interpreted as follows:
-
-\begin{description}
-\item[cap_vndr]
-        0x09; Identifies a vendor-specific capability.
-
-\item[cap_next]
-        Link to next capability in the capability list in the configuration space.
-
-\item[cap_len]
-        Length of the capability structure, including the whole of
-        struct virtio_pci_cap, and extra data if any.
-        This length might include padding, or fields unused by the driver.
-
-\item[cfg_type]
-        identifies the structure, according to the following table.
-
-\begin{lstlisting}
-/* Common configuration */
-#define VIRTIO_PCI_CAP_COMMON_CFG        1
-/* Notifications */
-#define VIRTIO_PCI_CAP_NOTIFY_CFG        2
-/* ISR Status */
-#define VIRTIO_PCI_CAP_ISR_CFG           3
-/* Device specific configuration */
-#define VIRTIO_PCI_CAP_DEVICE_CFG        4
-/* PCI configuration access */
-#define VIRTIO_PCI_CAP_PCI_CFG           5
-\end{lstlisting}
-
-        Any other value - reserved for future use. Drivers MUST
-        ignore any vendor-specific capability structure which has
-        a reserved cfg_type value.
-
-        More than one capability can identify the same structure - this makes it
-        possible for the device to expose multiple interfaces to drivers.  The order of
-        the capabilities in the capability list specifies the order of preference
-        suggested by the device; drivers SHOULD use the first interface that they can
-        support.  For example, on some hypervisors, notifications using IO accesses are
-        faster than memory accesses. In this case, hypervisor can expose two
-        capabilities with cfg_type set to VIRTIO_PCI_CAP_NOTIFY_CFG:
-        the first one addressing an I/O BAR, the second one addressing a memory BAR.
-        Driver will use the I/O BAR if I/O resources are available, and fall back on
-        memory BAR when I/O resources are unavailable.
-
-\item[bar]
-        values 0x0 to 0x5 specify a Base Address register (BAR) belonging to
-        the function located beginning at 10h in Configuration Space
-        and used to map the structure into Memory or I/O Space.
-        The BAR is permitted to be either 32-bit or 64-bit, it can map Memory Space
-        or I/O Space.
-
-        Any other value is reserved for future use. Drivers MUST
-        ignore any vendor-specific capability structure which has
-        a reserved bar value.
-
-\item[offset]
-        indicates where the structure begins relative to the base address associated
-        with the BAR.
-
-\item[length]
-        indicates the length of the structure.
-        This size might include padding, or fields unused by the driver.
-        Drivers SHOULD only map part of configuration structure
-        large enough for device operation.
-        For example, a future device might present a large structure size of several
-        MBytes.
-        As current devices never utilize structures larger than 4KBytes in size,
-        driver can limit the mapped structure size to e.g.
-        4KBytes to allow forward compatibility with such devices without loss of
-        functionality and without wasting resources.
-\end{description}
-
-If cfg_type is VIRTIO_PCI_CAP_NOTIFY_CFG this structure is immediately followed
-by additional fields:
-
-\begin{lstlisting}
-struct virtio_pci_notify_cap {
-        struct virtio_pci_cap cap;
-        le32 notify_off_multiplier; /* Multiplier for queue_notify_off. */
-};
-\end{lstlisting}
-
-\begin{description}
-\item[notify_off_multiplier]
-
-        Virtqueue offset multiplier, in bytes. Must be even and either a power of two, or 0.
-        Value 0x1 is reserved.
-        For a given virtqueue, the address to use for notifications is calculated as follows:
-
-        queue_notify_off * notify_off_multiplier + offset
-
-        If notify_off_multiplier is 0, all virtqueues use the same address in
-        the Notifications structure!
-\end{description}
-
-If cfg_type is VIRTIO_PCI_CAP_PCI_CFG the fields bar, offset and length are RW
-and this structure is immediately followed by an additional field:
-
-\begin{lstlisting}
-struct virtio_pci_cfg_cap {
-        __u8 pci_cfg_data[4]; /* Data for BAR access. */
-};
-\end{lstlisting}
-
-\begin{description}
-\item[pci_cfg_data]
-
-        This RW field allows an indirect access to any BAR on the
-        device using PCI configuration accesses.
-
-        The BAR to access is selected using the bar field.
-        The length of the access is specified by the length
-        field, which can be set to 1, 2 and 4.
-        The offset within the BAR is specified by the offset
-        field, which must be aligned to length bytes.
-
-        After this field is written by driver, the first length
-        bytes in pci_cfg_data are written at the selected
-        offset in the selected BAR.
-
-        When this field is read by driver, length bytes at the
-        selected offset in the selected BAR are read into pci_cfg_data.
-\end{description}
-
 \subparagraph{Legacy Interface: A Note on Device Layout Detection}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Device Initialization / Virtio Device Configuration Layout Detection / Legacy Interface: A Note on Device Layout Detection}
 
-Legacy drivers skipped  Device Layout Detection step, assuming legacy
+Legacy drivers skipped the Device Layout Detection step, assuming legacy
 configuration space in BAR0 in I/O space unconditionally.
 
 Legacy devices did not have the Virtio PCI Capability in their
@@ -1226,7 +1233,7 @@ and fail gracefully.
 
 Non-transitional devices, on a platform where a legacy driver for
 a legacy device with the same ID might have previously existed,
-must take the following steps to fail gracefully when a legacy
+MUST take the following steps to fail gracefully when a legacy
 driver attempts to drive them:
 
 \begin{enumerate}
@@ -1305,16 +1312,16 @@ device is defined as 4096 bytes.  Driver writes the physical address, divided
 by 4096 to the Queue Address field\footnote{The 4096 is based on the x86 page size, but it's also large
 enough to ensure that the separate parts of the virtqueue are on
 separate cache lines.
-}.
+}.  There was no mechanism to negotiate the queue size.
 
 \subsubsection{Notifying The Device}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Notifying The Device}
 
-Device notification occurs by writing the 16-bit virtqueue index
-of this virtqueue to the Queue Notify field.
+The driver notifies the device by writing the 16-bit virtqueue index
+of this virtqueue to the Queue Notify address.  See \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability} for how to calculate this address.
 
 \subsubsection{Virtqueue Interrupts From The Device}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Virtqueue Interrupts From The Device}
 
-If an interrupt is necessary:
+If an interrupt is necessary, the device SHOULD:
 
 \begin{itemize}
   \item If MSI-X capability is disabled:
@@ -1331,22 +1338,23 @@ If an interrupt is necessary:
       number.
 
     \item If Queue Vector field value is NO_VECTOR, no interrupt
-      message is requested for this event.
+      message is requested for this event, so the device MUST NOT
+      deliver an interrupt.
     \end{enumerate}
 \end{itemize}
 
-The driver interrupt handler should:
+The driver interrupt handler SHOULD:
 
 \begin{itemize}
   \item If MSI-X capability is disabled: read the ISR Status field,
   which will reset it to zero. If the lower bit is zero, the
   interrupt was not for this device. Otherwise, the driver
-  should look through the used rings of each virtqueue for the
+  SHOULD look through the used rings of all virtqueues for the
   device, to see if any progress has been made by the device
   which requires servicing.
 
   \item If MSI-X capability is enabled: look through the used rings of
-  each virtqueue mapped to the specific MSI-X vector for the
+  all virtqueues mapped to the specific MSI-X vector for the
   device, to see if any progress has been made by the device
   which requires servicing.
 \end{itemize}
@@ -1354,8 +1362,7 @@ The driver interrupt handler should:
 \subsubsection{Notification of Device Configuration Changes}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Notification of Device Configuration Changes}
 
 Some virtio PCI devices can change the device configuration
-state, as reflected in the virtio header in the PCI configuration
-space. In this case:
+state, as reflected in the device-specific region of the device. In this case:
 
 \begin{itemize}
   \item If MSI-X capability is disabled: an interrupt is delivered and
@@ -1364,12 +1371,13 @@ space. In this case:
   space.  Note that a single interrupt can indicate both that one
   or more virtqueue has been used and that the configuration
   space has changed: even if the config bit is set, virtqueues
-  must be scanned.
+  MUST be scanned.
 
   \item If MSI-X capability is enabled: an interrupt message is
   requested. The Configuration Vector field sets the MSI-X Table
   entry number to use. If Configuration Vector field value is
-  NO_VECTOR, no interrupt message is requested for this event.
+  NO_VECTOR, no interrupt message is requested for this event and
+  the device MUST NOT deliver an interrupt.
 \end{itemize}
 
 \section{Virtio Over MMIO}\label{sec:Virtio Transport Options / Virtio Over MMIO}