From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough Date: Wed, 04 Jul 2012 18:32:35 +0200 Message-ID: <4FF47023.7080206@redhat.com> References: <1341321577-24435-1-git-send-email-pbonzini@redhat.com> <1341419217.18786.3.camel@lappy> 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: Sasha Levin Return-path: Received: from mx1.redhat.com ([209.132.183.28]:43824 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751369Ab2GDQco (ORCPT ); Wed, 4 Jul 2012 12:32:44 -0400 In-Reply-To: <1341419217.18786.3.camel@lappy> Sender: kvm-owner@vger.kernel.org List-ID: 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