From mboxrd@z Thu Jan 1 00:00:00 1970 From: Asias He Subject: Re: [PATCH V3 3/3] virtio-blk: Add bio-based IO path for virtio-blk Date: Sat, 28 Jul 2012 10:38:41 +0800 Message-ID: <501350B1.7080908@redhat.com> References: <1342168731-11797-1-git-send-email-asias@redhat.com> <1342168731-11797-4-git-send-email-asias@redhat.com> <87d33initv.fsf@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <87d33initv.fsf@rustcorp.com.au> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Rusty Russell Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Paolo Bonzini , Christoph Hellwig List-Id: virtualization@lists.linuxfoundation.org On 07/27/2012 08:33 AM, Rusty Russell wrote: > On Fri, 13 Jul 2012 16:38:51 +0800, Asias He wrote: >> Add 'virtio_blk.use_bio=1' to kernel cmdline or 'modprobe virtio_blk >> use_bio=1' to enable ->make_request_fn() based I/O path. > > This patch conflicts with Paolo's Bonzini's 'virtio-blk: allow toggling > host cache between writeback and writethrough' which is also queued (see > linux-next). Rebased against Paolo's patch in V4. > I'm not sure what the correct behavior for bio & cacheflush is, if any. REQ_FLUSH is not supported in the bio path. > But as to the patch itself: it's a hack. > > 1) Leaving the guest's admin to turn on the switch is a terrible choice. > 2) The block layer should stop merging and sorting when a device is > fast, not the driver. > 3) I pointed out that slow disks have low IOPS, so why is this > conditional? Sure, more guest exits, but it's still a small number > for a slow device. > 4) The only case where we want merging is on a slow device when the host > isn't doing it. > > Now, despite this, I'm prepared to commit it. But in my mind it's a > hack: we should aim for use_bio to be based on a feature bit fed from > the host, and use the module parameter only if we want to override it. OK. A feature bit from host sound like a choice but a switch is also needed on host side. And for other OS, e.g. Windows, the bio thing does not apply at all. Anyway, I have to admit that adding a module parameter here is not the best choice. Let's think more. -- Asias