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

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/07/2015 09:01, Michael S. Tsirkin wrote:
> On Thu, Jul 02, 2015 at 12:41:04AM +0200, Paolo Bonzini wrote:
>> 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>
> 
> Please mention the issue ID in the commit log in the future.
> E.g. on a line by itself:

Ok.  It's in the subject though, which technically is the first line of
the commit log. :)

> VIRTIO-144
> 
>> ---
>>  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
>>      alignment.
>> +
>> +\item[VIRTIO_BLK_F_CONFIG_WCE (11)] Device can toggle its cache between writeback
>> +    and writethrough modes.
>>  \end{description}
>>  
>>  \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.
>>  \end{description}
>>  
>>  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.
>>  
>>  \begin{lstlisting}
>>  struct virtio_blk_config {
> 
> I'm afraid we can't make this change at this point - it's clearly
> incompatible with devices that implement cs01 unless they
> also implement the WCE bit.
> 
> If you see value in having a RO writeback field,
> just add a feature bit for that.

No, it's okay to remove the RO capability.  I'll send v2.

> 
>> @@ -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;
>>  };
>>  \end{lstlisting}
>>  
>> @@ -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.}.
>>  \end{enumerate}
> 
> 
> Could you add some conformance statements for this as well
> please?

The above footnote is a consequence of the conformance statements in the
"Device Operation" section.  (Hence putting it in a footnote only).

>>  \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.
>> +
> 
> Maybe clarify what happens if it doesn't "Failure to do so might
> lead to data loss"?

Okay.

Paolo