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Subject: [PATCH] virtio-blk: restore VIRTIO_BLK_F_CONFIG_WCE (VIRTIO-144)

VIRTIO_BLK_F_CONFIG_WCE is important in order to achieve good performance
(up to 2x, though more realistically +30-40%) in latency-bound workloads.
However, it was removed by mistake together with VIRTIO_BLK_F_FLUSH.

Restore it, clarifying the definition of writeback and writethrough cache
according to what is commonly done by high-end storage.  In addition, fix
two occurrences where the driver was mentioned rather than the device, and
clarify the influence of the legacy VIRTIO_BLK_F_FLUSH feature on caching.

There is a difference between the legacy and virtio1 behavior of
VIRTIO_BLK_F_CONFIG_WCE.  In legacy devices, the writeback field was
only available if VIRTIO_BLK_F_CONFIG_WCE was negotiated, while now
it is always available.  However, the VIRTIO_BLK_F_CONFIG_WCE feature
remains and describes whether the field is writable.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
 content.tex | 45 ++++++++++++++++++++++++++++++++-------------
 1 file changed, 32 insertions(+), 13 deletions(-)

diff --git a/content.tex b/content.tex
index 1efdcc8..7c00fd7 100644
--- a/content.tex
+++ b/content.tex
@@ -3725,6 +3725,9 @@ device except where noted.
 \item[VIRTIO_BLK_F_TOPOLOGY (10)] Device exports information on optimal I/O
+\item[VIRTIO_BLK_F_CONFIG_WCE (11)] Device can toggle its cache between writeback
+    and writethrough modes.
 \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Block Device / Feature bits / Legacy Interface: Feature bits}
@@ -3735,9 +3738,6 @@ device except where noted.
 \item[VIRTIO_BLK_F_SCSI (7)] Device supports scsi packet commands.
 \item[VIRTIO_BLK_F_FLUSH (9)] Cache flush command support.
-\item[VIRTIO_BLK_F_CONFIG_WCE (11)] Device can toggle its cache between writeback
-    and writethrough modes.
 VIRTIO_BLK_F_FLUSH was also called VIRTIO_BLK_F_WCE: Legacy drivers
@@ -3746,9 +3746,9 @@ VIRTIO_BLK_T_FLUSH commands.
 \subsection{Device configuration layout}\label{sec:Device Types / Block Device / Device configuration layout}
-The \field{capacity} of the device (expressed in 512-byte sectors) is always
-present. The availability of the others all depend on various feature
-bits as indicated above.
+The \field{capacity} of the device (expressed in 512-byte sectors) and
+caching mode (\field{writeback}) are always present. The availability
+of the others all depend on various feature bits as indicated above.
 struct virtio_blk_config {
@@ -3771,7 +3771,7 @@ struct virtio_blk_config {
                 // optimal (suggested maximum) I/O size in blocks
                 le32 opt_io_size;
         } topology;
-        u8 reserved;
+        u8 writeback;
@@ -3801,15 +3801,20 @@ according to the native endian of the guest rather than
   \field{topology} struct can be read to determine the physical block size and optimal
   I/O lengths for the driver to use. This also does not affect the units
   in the protocol, only performance.
+\item The cache mode can be read from \field{writeback}.  0 corresponds
+  to a writethrough cache, 1 to a writeback cache\footnote{Consistent
+    with \ref{sec:Device Types / Block Device / Device Operation},
+    a writethrough cache can be defined broadly as a cache that commits
+    writes to non-volatile storage before reporting their completion.
+    For example, a battery-backed writeback cache actually counts as
+    writethrough according to this definition.}.
 \subsubsection{Legacy Interface: Device Initialization}\label{sec:Device Types / Block Device / Device Initialization / Legacy Interface: Device Initialization}
-The \field{reserved} field used to be called \field{writeback}.  If the
-VIRTIO_BLK_F_CONFIG_WCE feature is offered, the cache mode can be
-read from \field{writeback}; the
-driver can also write to the field in order to toggle the cache
-between writethrough (0) and writeback (1) mode.  If the feature is
+For legacy devices, the \field{writeback} field is only available if
+the VIRTIO_BLK_F_CONFIG_WCE feature is offered.  If the feature is
 not available, the driver can instead look at the result of
 negotiating VIRTIO_BLK_F_FLUSH: the cache will be in writeback mode
 after reset if and only if VIRTIO_BLK_F_FLUSH is negotiated.
@@ -3871,9 +3876,19 @@ A device MUST set the \field{status} byte to VIRTIO_BLK_S_IOERR
 for a write request if the VIRTIO_BLK_F_RO feature if offered, and MUST NOT
 write any data.
-Upon receipt of a VIRTIO_BLK_T_FLUSH request, the driver SHOULD ensure
+Upon receipt of a VIRTIO_BLK_T_FLUSH request, the device SHOULD ensure
 that any writes which were completed are committed to non-volatile storage.
+If the \field{writeback} field in configuration space is 0, the device
+SHOULD ensure that all writes are committed to non-volatile storage before
+reporting completion.
+If the VIRTIO_BLK_F_CONFIG_WCE feature is negotiated, the driver MAY
+switch to writethrough or writeback mode by writing respectively 0 and
+1 to the \field{writeback} field.  After writing a 0 to \field{writeback},
+the driver MUST NOT assume that writes submitted while the field was 1 have
+been committed to non-volatile storage.
 \subsubsection{Legacy Interface: Device Operation}\label{sec:Device Types / Block Device / Device Operation / Legacy Interface: Device Operation}
 When using the legacy interface, transitional devices and drivers
 MUST format the fields in struct virtio_blk_req
@@ -3884,6 +3899,10 @@ The \field{reserved} field was previously called \field{ioprio}.  \field{ioprio}
 is a hint about the relative priorities of requests to the device:
 higher numbers indicate more important requests.
+If the \field{VIRTIO_BLK_F_FLUSH} feature is not negotiated, the device
+SHOULD ensure that all writes are committed to non-volatile storage before
+reporting completion.
 #define VIRTIO_BLK_T_FLUSH_OUT    5

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