From: Paolo Bonzini <pbonzini@redhat.com>
To: Sasha Levin <levinsasha928@gmail.com>
Cc: linux-kernel@vger.kernel.org, rusty@rustcorp.com.au, kvm@vger.kernel.org
Subject: Re: [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough
Date: Wed, 04 Jul 2012 18:32:35 +0200 [thread overview]
Message-ID: <4FF47023.7080206@redhat.com> (raw)
In-Reply-To: <1341419217.18786.3.camel@lappy>
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
next prev parent reply other threads:[~2012-07-04 16:32 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-07-03 13:19 [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough Paolo Bonzini
2012-07-04 5:55 ` Rusty Russell
2012-07-04 6:24 ` Rusty Russell
2012-07-04 15:13 ` Michael S. Tsirkin
2012-07-04 15:42 ` Michael S. Tsirkin
2012-07-04 15:42 ` Michael S. Tsirkin
2012-07-04 15:54 ` Paolo Bonzini
2012-07-04 16:02 ` Michael S. Tsirkin
2012-07-04 16:02 ` Michael S. Tsirkin
2012-07-04 16:08 ` Paolo Bonzini
2012-07-04 16:08 ` Paolo Bonzini
2012-07-04 21:30 ` Michael S. Tsirkin
2012-07-04 21:30 ` Michael S. Tsirkin
2012-07-05 6:45 ` Paolo Bonzini
2012-07-05 6:45 ` Paolo Bonzini
2012-07-04 15:54 ` Paolo Bonzini
2012-07-04 16:26 ` Sasha Levin
2012-07-04 16:32 ` Paolo Bonzini [this message]
2012-07-04 21:11 ` Sasha Levin
2012-07-05 6:47 ` Paolo Bonzini
2012-07-05 7:02 ` Sasha Levin
2012-07-08 23:45 ` Rusty Russell
2012-07-25 1:02 ` Rusty Russell
2012-07-05 18:39 ` Badari Pulavarty
2012-07-06 6:47 ` Paolo Bonzini
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