linux-kernel - Re: [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough

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Date:	Wed, 04 Jul 2012 18:32:35 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
CC:	linux-kernel@...r.kernel.org, rusty@...tcorp.com.au,
	kvm@...r.kernel.org
Subject: Re: [PATCH] virtio-blk: allow toggling host cache between writeback
 and writethrough

Il 04/07/2012 18:26, Sasha Levin ha scritto:
> On Tue, 2012-07-03 at 15:19 +0200, Paolo Bonzini wrote:
>> diff --git a/include/linux/virtio_blk.h b/include/linux/virtio_blk.h
>> index e0edb40..18a1027 100644
>> --- a/include/linux/virtio_blk.h
>> +++ b/include/linux/virtio_blk.h
>> @@ -37,8 +37,9 @@
>>  #define VIRTIO_BLK_F_RO                5       /* Disk is read-only */
>>  #define VIRTIO_BLK_F_BLK_SIZE  6       /* Block size of disk is available*/
>>  #define VIRTIO_BLK_F_SCSI      7       /* Supports scsi command passthru */
>> -#define VIRTIO_BLK_F_FLUSH     9       /* Cache flush command support */
>> +#define VIRTIO_BLK_F_WCE       9       /* Writeback mode enabled after reset */
>>  #define VIRTIO_BLK_F_TOPOLOGY  10      /* Topology information is available */
>> +#define VIRTIO_BLK_F_CONFIG_WCE        11      /* Writeback mode available in config */ 
> 
> Wouldn't this change break any usermode code that implements virtio-blk?

No, the change is really just clarifying the existing spec, and
mandating that virtio-blk implementations follow certain assumptions of
the Linux driver.

In particular, the Linux driver is already assuming that the host
exposes VIRTIO_BLK_F_FLUSH if and only if it exposes a volatile write
cache.  This works because if you have a writeback cache, but provide no
way to flush it, the guest driver really cannot do anything about it
anyway.  Might as well treat it as writethrough, i.e. blk_queue_flush(q, 0).

QEMU in fact has already behaved like that, and even called the flag
VIRTIO_BLK_F_WCACHE instead of VIRRTIO_BLK_F_FLUSH.

Paolo
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