From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sasha Levin Subject: Re: [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough Date: Wed, 04 Jul 2012 23:11:25 +0200 Message-ID: <1341436285.18786.6.camel@lappy> References: <1341321577-24435-1-git-send-email-pbonzini@redhat.com> <1341419217.18786.3.camel@lappy> <4FF47023.7080206@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, rusty@rustcorp.com.au, kvm@vger.kernel.org To: Paolo Bonzini Return-path: In-Reply-To: <4FF47023.7080206@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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.