Il 03/10/2013 07:05, Rusty Russell ha scritto:
> Paolo Bonzini <> writes:
>> Il 02/10/2013 02:50, Rusty Russell ha scritto:
>>>>> I would make this even stronger, like "It is undefined whether a disk
>>>>> operates in writeback or writethrough mode after a guest announces lack
>>>>> of support for VIRTIO_BLK_F_FLUSH".
>>> At this point it's documenting historical behavior, rather than
>>> proscribing it. How about we simply note:
>>>
>>> Some older legacy devices did not operate in writethrough mode even
>>> after a guest announced lack of support for VIRTIO_BLK_F_FLUSH.
>>
>> That's fine.
>>
>> Paolo
>
> Here's the final patch then:
>
> commit ff7486a8699c06743473c0078e9b511275d65306
> Author: Rusty Russell <>
> Date: Tue Oct 1 14:53:17 2013 +0930
>
> virtio_blk: deprecate flush.
>
> This is partially a port of Paolo Bonzini <>'s
> WCE spec changes, but puts them in the legacy section. It reserves
> the writeback field for compatibility.
>
> Flush is now required.
>
> See: https://lists.oasis-open.org/archives/virtio-comment/201308/msg00078.html
>
> Signed-off-by: Rusty Russell <>
>
> diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
> index 5efeba7..a9c0cce 100644
> --- a/virtio-v1.0-wd01-part1-specification.txt
> +++ b/virtio-v1.0-wd01-part1-specification.txt
> @@ -1817,8 +1817,6 @@ device except where noted.
>
> VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”.
>
> - VIRTIO_BLK_F_FLUSH (9) Cache flush command support.
> -
> VIRTIO_BLK_F_TOPOLOGY (10) Device exports information on optimal I/O
> alignment.
>
> @@ -1843,6 +1841,7 @@ device except where noted.
> u16 min_io_size;
> u32 opt_io_size;
> } topology;
> + u8 reserved;
> };
>
> 2.4.2.3.1 Legacy Interface: Feature bits
> @@ -1851,6 +1850,15 @@ device except where noted.
>
> VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands.
>
> + VIRTIO_BLK_F_FLUSH (9) Cache flush command support.
> +
> + 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
> +should only negotiate this feature if they are capable of sending
> +VIRTIO_BLK_T_FLUSH commands.
> +
> 2.4.2.4. Device Initialization
> -----------------------------
>
> @@ -1872,6 +1880,21 @@ device except where noted.
> I/O lengths for the driver to use. This also does not affect the units
> in the protocol, only performance.
>
> +2.4.2.4.1. Legacy Interface: Device Initialization
> +-----------------------------
> +
> +The reserved field used to be called writeback. If the
> +VIRTIO_BLK_F_CONFIG_FLUSH feature is offered, the cache mode should be
CONFIG_WCE?
> +read from the writeback field of the configuration if available; 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
> +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.
> +
> +Some older legacy devices did not operate in writethrough mode even
> +after a guest announced lack of support for VIRTIO_BLK_F_FLUSH.
> +
> 2.4.2.5. Device Operation
> ------------------------
>
> @@ -1907,6 +1930,9 @@ error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host:
> #define VIRTIO_BLK_S_IOERR 1
> #define VIRTIO_BLK_S_UNSUPP 2
>
> +Any writes completed before the submission of the flush command should
> +be committed to permanent storage by the device.
s/permanent/non-volatile/
Putting them in a non-volatile cache is fine.
Paolo
> +
> 2.4.2.5.1 Legacy Interface: Device Operation
> ------------------------
> The 'reserved' field was previously called ioprio. The ioprio field
>
>
> ---------------------------------------------------------------------
> To unsubscribe from this mail list, you must leave the OASIS TC that
> generates this mail. Follow this link to all your TCs in OASIS at:
> https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php
>
