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

From: Sasha Levin <levinsasha928@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.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 23:11:25 +0200	[thread overview]
Message-ID: <1341436285.18786.6.camel@lappy> (raw)
In-Reply-To: <4FF47023.7080206@redhat.com>

On Wed, 2012-07-04 at 18:32 +0200, Paolo Bonzini wrote:
> 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.

There are two things going on here:
 1. Rename VIRTIO_BLK_F_FLUSH to VIRTIO_BLK_F_WCE
 2. Add a new VIRTIO_BLK_F_CONFIG_WCE

I'm concerned that the first change is going to break compilation for
any code that included linux/virtio-blk.h and used VIRTIO_BLK_F_FLUSH.