Re: [virtio] Re: [virtio-dev] Re: [virtio-comment] Problems with VIRTIO-4 and writeback only disks

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 03, 2013 at 02:35:04PM +0930, Rusty Russell wrote:
> Paolo Bonzini <pbonzini@redhat.com> 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 <rusty@au1.ibm.com>
> Date:   Tue Oct 1 14:53:17 2013 +0930
> 
>     virtio_blk: deprecate flush.
>     
>     This is partially a port of Paolo Bonzini <pbonzini@redhat.com>'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 <rusty@au1.ibm.com>

Can you do git format-patch -v2/git send-email for patch
submission please?
Makes it much easier to spot when there's a new proposal,
and git am produces a messy log if you don't.


> 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
> +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.
> +
>  2.4.2.5.1 Legacy Interface: Device Operation
>  ------------------------
>  The 'reserved' field was previously called ioprio.  The ioprio field