[PATCH v4] virtio-blk: Add support for "Force Unit Access" writes

[PATCH v4] virtio-blk: Add support for "Force Unit Access" writes

[Alberto Faria]

Add a VIRTIO_BLK_F_REQ_FLAGS feature bit converting the current
`virtio_blk_req::reserved` field into a `flags` bit field, which can be
used to modify the behavior of an entire request. The meaning of each
bit depends on the request type.

Define a single VIRTIO_BLK_REQ_FLAG_OUT_FUA bit as signaling that a
VIRTIO_BLK_T_OUT request should be a "Force Unit Access" (FUA) write,
i.e., should become stable once the request completes. FUA writes enable
better performance compared to the alternative of waiting for a write to
complete and subsequently submitting a flush.

Also add a VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA feature bit indicating device
support for the aforementioned FUA bit.

This approach allows for future expansion to other request-level flags
and allows the same flag bit to be used for different purposes on
different request types. The VIRTIO_BLK_F_REQ_FLAGS feature bit ensures
compatibility with legacy devices/drivers that interpret the
previously-`reserved` field as a priority indicator.

Signed-off-by: Alberto Faria <afaria@redhat.com>
---

v4:
- Have the semantics of each request flag depend on the request type, as
  suggested by Stefan.
- Some other smaller rewordings suggested by Stefan.

v3:
- Changed to a more future-proof approach somewhat similar to what was
  suggested by Stefan.
- Included a brief rationale for the introduction of FUA write requests,
  as suggested by Michael.

v2:
- Redefine VIRTIO_BLK_T_OUT_FUA to 27 since 15 is already in use.
- Clarify that the cache mode has no impact on VIRTIO_BLK_T_OUT_FUA
  semantics.
- Allow drivers to negotiate VIRTIO_BLK_F_OUT_FUA even if they are
  incapable of sending VIRTIO_BLK_T_OUT_FUA commands.

 device-types/blk/description.tex | 43 +++++++++++++++++++++++++++++---
 1 file changed, 39 insertions(+), 4 deletions(-)

diff --git a/device-types/blk/description.tex b/device-types/blk/description.tex
index 2712ada..3b3a4e7 100644
--- a/device-types/blk/description.tex
+++ b/device-types/blk/description.tex
@@ -66,6 +66,13 @@ \subsection{Feature bits}\label{sec:Device Types / Block Device / Feature bits}
 	(ZNS). For brevity, these standard documents are referred as "ZBD standards"
 	from this point on in the text.
 
+\item[VIRTIO_BLK_F_REQ_FLAGS (18)] Device can interpret the \field{flags}
+    bitfield in the \field{virtio_blk_req} structure.
+
+\item[VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA (19)] Device supports the
+    VIRTIO_BLK_REQ_FLAG_OUT_FUA flag in the \field{flags} bitfield of the
+    \field{virtio_blk_req} structure for VIRTIO_BLK_T_OUT requests.
+
 \end{description}
 
 \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Block Device / Feature bits / Legacy Interface: Feature bits}
@@ -317,6 +324,9 @@ \subsection{Device Initialization}\label{sec:Device Types / Block Device / Devic
     driver SHOULD ignore all other fields in \field{zoned}.
 \end{itemize}
 
+The driver MUST NOT negotiate VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA without
+VIRTIO_BLK_F_REQ_FLAGS.
+
 \devicenormative{\subsubsection}{Device Initialization}{Device Types / Block Device / Device Initialization}
 
 Devices SHOULD always offer VIRTIO_BLK_F_FLUSH, and MUST offer it
@@ -402,6 +412,9 @@ \subsection{Device Initialization}\label{sec:Device Types / Block Device / Devic
 \item the device MUST initialize padding bytes \field{unused2} to 0.
 \end{itemize}
 
+The device MUST NOT acknowledge FEATURES_OK if the driver sets
+VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA without VIRTIO_BLK_F_REQ_FLAGS.
+
 \subsubsection{Legacy Interface: Device Initialization}\label{sec:Device Types / Block Device / Device Initialization / Legacy Interface: Device Initialization}
 
 Because legacy devices do not have FEATURES_OK, transitional devices
@@ -434,7 +447,7 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
 \begin{lstlisting}
 struct virtio_blk_req {
         le32 type;
-        le32 reserved;
+        le32 flags;
         le64 sector;
         u8 data[];
         u8 status;
@@ -459,6 +472,16 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
 #define VIRTIO_BLK_T_SECURE_ERASE   14
 \end{lstlisting}
 
+The \field{flags} bitfield is ignored by the device unless
+VIRTIO_BLK_F_REQ_FLAGS is negotiated, in which case each bit's meaning depends
+on the request type. The following flags are currently defined (the numeric
+value is the bit index in the \field{flags} bitfield):
+
+\begin{description}
+\item[VIRTIO_BLK_REQ_FLAG_OUT_FUA (0) for VIRTIO_BLK_T_OUT requests] Force Unit
+    Access (FUA) flag.
+\end{description}
+
 The \field{sector} number indicates the offset (multiplied by 512) where
 the read or write is to occur. This field is unused and set to 0 for
 commands other than read, write and some zone operations.
@@ -873,6 +896,11 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
 
 A driver SHOULD accept the VIRTIO_BLK_F_RO feature if offered.
 
+If VIRTIO_BLK_F_REQ_FLAGS is negotiated, a driver MUST NOT set a bit in
+\field{flags} (e.g., VIRTIO_BLK_REQ_FLAG_OUT_FUA) unless the corresponding
+feature (e.g., VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA) for the request type in question
+(e.g., VIRTIO_BLK_T_OUT) is negotiated.
+
 A driver MUST set \field{sector} to 0 for a VIRTIO_BLK_T_FLUSH request.
 A driver SHOULD NOT include any data in a VIRTIO_BLK_T_FLUSH request.
 
@@ -1000,14 +1028,21 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
   \field{writeback} field in configuration space was 0 \textbf{all the time between
   the submission of the write and its completion};
 
-\item\label{item:flush3} a VIRTIO_BLK_T_FLUSH request is sent \textbf{after the write is
+\item\label{item:flush3} the VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA feature was
+  negotiated and the VIRTIO_BLK_REQ_FLAG_OUT_FUA bit in \field{flags} was set in
+  the write request (regardless of whether the VIRTIO_BLK_F_FLUSH or
+  VIRTIO_BLK_F_CONFIG_WCE features were negotiated, and regardless of the
+  current cache mode as expressed by the value of the \field{writeback} field in
+  configuration space).
+
+\item\label{item:flush4} a VIRTIO_BLK_T_FLUSH request is sent \textbf{after the write is
   completed} and is completed itself.
 \end{enumerate}
 
 If the device is backed by persistent storage, the device MUST ensure that
 stable writes are committed to it, before reporting completion of the write
-(cases~\ref{item:flush1} and~\ref{item:flush2}) or the flush
-(case~\ref{item:flush3}).  Failure to do so can cause data loss
+(cases~\ref{item:flush1}, \ref{item:flush2} and~\ref{item:flush3}) or the flush
+(case~\ref{item:flush4}).  Failure to do so can cause data loss
 in case of a crash.
 
 If the driver changes \field{writeback} between the submission of the write
-- 
2.51.1

Re: [PATCH v4] virtio-blk: Add support for "Force Unit Access" writes

[Stefan Hajnoczi]

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On Wed, Nov 12, 2025 at 05:51:13AM +0000, Alberto Faria wrote:
> Add a VIRTIO_BLK_F_REQ_FLAGS feature bit converting the current
> `virtio_blk_req::reserved` field into a `flags` bit field, which can be
> used to modify the behavior of an entire request. The meaning of each
> bit depends on the request type.
> 
> Define a single VIRTIO_BLK_REQ_FLAG_OUT_FUA bit as signaling that a
> VIRTIO_BLK_T_OUT request should be a "Force Unit Access" (FUA) write,
> i.e., should become stable once the request completes. FUA writes enable
> better performance compared to the alternative of waiting for a write to
> complete and subsequently submitting a flush.
> 
> Also add a VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA feature bit indicating device
> support for the aforementioned FUA bit.
> 
> This approach allows for future expansion to other request-level flags
> and allows the same flag bit to be used for different purposes on
> different request types. The VIRTIO_BLK_F_REQ_FLAGS feature bit ensures
> compatibility with legacy devices/drivers that interpret the
> previously-`reserved` field as a priority indicator.
> 
> Signed-off-by: Alberto Faria <afaria@redhat.com>
> ---
> 
> v4:
> - Have the semantics of each request flag depend on the request type, as
>   suggested by Stefan.
> - Some other smaller rewordings suggested by Stefan.
> 
> v3:
> - Changed to a more future-proof approach somewhat similar to what was
>   suggested by Stefan.
> - Included a brief rationale for the introduction of FUA write requests,
>   as suggested by Michael.
> 
> v2:
> - Redefine VIRTIO_BLK_T_OUT_FUA to 27 since 15 is already in use.
> - Clarify that the cache mode has no impact on VIRTIO_BLK_T_OUT_FUA
>   semantics.
> - Allow drivers to negotiate VIRTIO_BLK_F_OUT_FUA even if they are
>   incapable of sending VIRTIO_BLK_T_OUT_FUA commands.
> 
>  device-types/blk/description.tex | 43 +++++++++++++++++++++++++++++---
>  1 file changed, 39 insertions(+), 4 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

Please see here on how to bring this issue to a vote:
https://github.com/oasis-tcs/virtio-spec/tree/master?tab=readme-ov-file#use-of-github-issues

Thanks,
Stefan

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Re: [PATCH v4] virtio-blk: Add support for "Force Unit Access" writes

[Alberto Faria]

Could please open a voting ballot for these changes?

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/237

Thanks,
Alberto


On Wed, Nov 12, 2025 at 5:51 AM Alberto Faria <afaria@redhat.com> wrote:
>
> Add a VIRTIO_BLK_F_REQ_FLAGS feature bit converting the current
> `virtio_blk_req::reserved` field into a `flags` bit field, which can be
> used to modify the behavior of an entire request. The meaning of each
> bit depends on the request type.
>
> Define a single VIRTIO_BLK_REQ_FLAG_OUT_FUA bit as signaling that a
> VIRTIO_BLK_T_OUT request should be a "Force Unit Access" (FUA) write,
> i.e., should become stable once the request completes. FUA writes enable
> better performance compared to the alternative of waiting for a write to
> complete and subsequently submitting a flush.
>
> Also add a VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA feature bit indicating device
> support for the aforementioned FUA bit.
>
> This approach allows for future expansion to other request-level flags
> and allows the same flag bit to be used for different purposes on
> different request types. The VIRTIO_BLK_F_REQ_FLAGS feature bit ensures
> compatibility with legacy devices/drivers that interpret the
> previously-`reserved` field as a priority indicator.
>
> Signed-off-by: Alberto Faria <afaria@redhat.com>
> ---
>
> v4:
> - Have the semantics of each request flag depend on the request type, as
>   suggested by Stefan.
> - Some other smaller rewordings suggested by Stefan.
>
> v3:
> - Changed to a more future-proof approach somewhat similar to what was
>   suggested by Stefan.
> - Included a brief rationale for the introduction of FUA write requests,
>   as suggested by Michael.
>
> v2:
> - Redefine VIRTIO_BLK_T_OUT_FUA to 27 since 15 is already in use.
> - Clarify that the cache mode has no impact on VIRTIO_BLK_T_OUT_FUA
>   semantics.
> - Allow drivers to negotiate VIRTIO_BLK_F_OUT_FUA even if they are
>   incapable of sending VIRTIO_BLK_T_OUT_FUA commands.
>
>  device-types/blk/description.tex | 43 +++++++++++++++++++++++++++++---
>  1 file changed, 39 insertions(+), 4 deletions(-)
>
> diff --git a/device-types/blk/description.tex b/device-types/blk/description.tex
> index 2712ada..3b3a4e7 100644
> --- a/device-types/blk/description.tex
> +++ b/device-types/blk/description.tex
> @@ -66,6 +66,13 @@ \subsection{Feature bits}\label{sec:Device Types / Block Device / Feature bits}
>         (ZNS). For brevity, these standard documents are referred as "ZBD standards"
>         from this point on in the text.
>
> +\item[VIRTIO_BLK_F_REQ_FLAGS (18)] Device can interpret the \field{flags}
> +    bitfield in the \field{virtio_blk_req} structure.
> +
> +\item[VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA (19)] Device supports the
> +    VIRTIO_BLK_REQ_FLAG_OUT_FUA flag in the \field{flags} bitfield of the
> +    \field{virtio_blk_req} structure for VIRTIO_BLK_T_OUT requests.
> +
>  \end{description}
>
>  \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Block Device / Feature bits / Legacy Interface: Feature bits}
> @@ -317,6 +324,9 @@ \subsection{Device Initialization}\label{sec:Device Types / Block Device / Devic
>      driver SHOULD ignore all other fields in \field{zoned}.
>  \end{itemize}
>
> +The driver MUST NOT negotiate VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA without
> +VIRTIO_BLK_F_REQ_FLAGS.
> +
>  \devicenormative{\subsubsection}{Device Initialization}{Device Types / Block Device / Device Initialization}
>
>  Devices SHOULD always offer VIRTIO_BLK_F_FLUSH, and MUST offer it
> @@ -402,6 +412,9 @@ \subsection{Device Initialization}\label{sec:Device Types / Block Device / Devic
>  \item the device MUST initialize padding bytes \field{unused2} to 0.
>  \end{itemize}
>
> +The device MUST NOT acknowledge FEATURES_OK if the driver sets
> +VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA without VIRTIO_BLK_F_REQ_FLAGS.
> +
>  \subsubsection{Legacy Interface: Device Initialization}\label{sec:Device Types / Block Device / Device Initialization / Legacy Interface: Device Initialization}
>
>  Because legacy devices do not have FEATURES_OK, transitional devices
> @@ -434,7 +447,7 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
>  \begin{lstlisting}
>  struct virtio_blk_req {
>          le32 type;
> -        le32 reserved;
> +        le32 flags;
>          le64 sector;
>          u8 data[];
>          u8 status;
> @@ -459,6 +472,16 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
>  #define VIRTIO_BLK_T_SECURE_ERASE   14
>  \end{lstlisting}
>
> +The \field{flags} bitfield is ignored by the device unless
> +VIRTIO_BLK_F_REQ_FLAGS is negotiated, in which case each bit's meaning depends
> +on the request type. The following flags are currently defined (the numeric
> +value is the bit index in the \field{flags} bitfield):
> +
> +\begin{description}
> +\item[VIRTIO_BLK_REQ_FLAG_OUT_FUA (0) for VIRTIO_BLK_T_OUT requests] Force Unit
> +    Access (FUA) flag.
> +\end{description}
> +
>  The \field{sector} number indicates the offset (multiplied by 512) where
>  the read or write is to occur. This field is unused and set to 0 for
>  commands other than read, write and some zone operations.
> @@ -873,6 +896,11 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
>
>  A driver SHOULD accept the VIRTIO_BLK_F_RO feature if offered.
>
> +If VIRTIO_BLK_F_REQ_FLAGS is negotiated, a driver MUST NOT set a bit in
> +\field{flags} (e.g., VIRTIO_BLK_REQ_FLAG_OUT_FUA) unless the corresponding
> +feature (e.g., VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA) for the request type in question
> +(e.g., VIRTIO_BLK_T_OUT) is negotiated.
> +
>  A driver MUST set \field{sector} to 0 for a VIRTIO_BLK_T_FLUSH request.
>  A driver SHOULD NOT include any data in a VIRTIO_BLK_T_FLUSH request.
>
> @@ -1000,14 +1028,21 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
>    \field{writeback} field in configuration space was 0 \textbf{all the time between
>    the submission of the write and its completion};
>
> -\item\label{item:flush3} a VIRTIO_BLK_T_FLUSH request is sent \textbf{after the write is
> +\item\label{item:flush3} the VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA feature was
> +  negotiated and the VIRTIO_BLK_REQ_FLAG_OUT_FUA bit in \field{flags} was set in
> +  the write request (regardless of whether the VIRTIO_BLK_F_FLUSH or
> +  VIRTIO_BLK_F_CONFIG_WCE features were negotiated, and regardless of the
> +  current cache mode as expressed by the value of the \field{writeback} field in
> +  configuration space).
> +
> +\item\label{item:flush4} a VIRTIO_BLK_T_FLUSH request is sent \textbf{after the write is
>    completed} and is completed itself.
>  \end{enumerate}
>
>  If the device is backed by persistent storage, the device MUST ensure that
>  stable writes are committed to it, before reporting completion of the write
> -(cases~\ref{item:flush1} and~\ref{item:flush2}) or the flush
> -(case~\ref{item:flush3}).  Failure to do so can cause data loss
> +(cases~\ref{item:flush1}, \ref{item:flush2} and~\ref{item:flush3}) or the flush
> +(case~\ref{item:flush4}).  Failure to do so can cause data loss
>  in case of a crash.
>
>  If the driver changes \field{writeback} between the submission of the write
> --
> 2.51.1
>

Re: [PATCH v4] virtio-blk: Add support for "Force Unit Access" writes

[Stefan Hajnoczi]

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On Mon, May 11, 2026 at 09:48:32AM -0400, Stefan Hajnoczi wrote:
[Resent from my virtio-comment list subscriber email address]

Hi Michael,
I don't think this issue was ever voted on?

I have verified that the spec patch still applies cleanly to the master branch.

Please begin the vote.

Thanks,
Stefan

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Re: [PATCH v4] virtio-blk: Add support for "Force Unit Access" writes

[Matias Ezequiel Vara Larsen]

On Mon, May 11, 2026 at 7:55 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Mon, May 11, 2026 at 09:48:32AM -0400, Stefan Hajnoczi wrote:
> [Resent from my virtio-comment list subscriber email address]
>
> Hi Michael,
> I don't think this issue was ever voted on?
>
> I have verified that the spec patch still applies cleanly to the master branch.
>
> Please begin the vote.
>

Apologies for the delay, I just created the ballot.

Matias