From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:52812) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TioVc-0007ID-Aw for qemu-devel@nongnu.org; Wed, 12 Dec 2012 10:46:16 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TioVT-0001wV-Ha for qemu-devel@nongnu.org; Wed, 12 Dec 2012 10:46:08 -0500 Received: from mx1.redhat.com ([209.132.183.28]:60934) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TioVT-0001wF-AQ for qemu-devel@nongnu.org; Wed, 12 Dec 2012 10:45:59 -0500 Date: Wed, 12 Dec 2012 17:49:06 +0200 From: "Michael S. Tsirkin" Message-ID: <20121212154906.GB17446@redhat.com> References: <1355144985-12897-1-git-send-email-stefanha@redhat.com> <1355144985-12897-4-git-send-email-stefanha@redhat.com> <20121211141340.GA18753@redhat.com> <20121211154258.GA20251@redhat.com> <87d2ygpn1f.fsf@codemonkey.ws> <20121211180956.GB9257@redhat.com> <20121212153421.GA16956@stefanha-thinkpad.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121212153421.GA16956@stefanha-thinkpad.redhat.com> Subject: Re: [Qemu-devel] [PATCH v6 03/12] dataplane: add host memory mapping code List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , Anthony Liguori , qemu-devel , Blue Swirl , Khoa Huynh , Stefan Hajnoczi , Paolo Bonzini , Asias He On Wed, Dec 12, 2012 at 04:34:21PM +0100, Stefan Hajnoczi wrote: > On Tue, Dec 11, 2012 at 08:09:56PM +0200, Michael S. Tsirkin wrote: > > On Tue, Dec 11, 2012 at 10:32:28AM -0600, Anthony Liguori wrote: > > > "Michael S. Tsirkin" writes: > > > > > > > On Tue, Dec 11, 2012 at 04:27:49PM +0100, Stefan Hajnoczi wrote: > > > >> On Tue, Dec 11, 2012 at 3:13 PM, Michael S. Tsirkin wrote: > > > >> > On Mon, Dec 10, 2012 at 02:09:36PM +0100, Stefan Hajnoczi wrote: > > > >> >> The data plane thread needs to map guest physical addresses to host > > > >> >> pointers. Normally this is done with cpu_physical_memory_map() but the > > > >> >> function assumes the global mutex is held. The data plane thread does > > > >> >> not touch the global mutex and therefore needs a thread-safe memory > > > >> >> mapping mechanism. > > > >> >> > > > >> >> Hostmem registers a MemoryListener similar to how vhost collects and > > > >> >> pushes memory region information into the kernel. There is a > > > >> >> fine-grained lock on the regions list which is held during lookup and > > > >> >> when installing a new regions list. > > > >> > > > > >> > Can we export and reuse the vhost code for this? > > > >> > I think you will find this advantageous when you add migration > > > >> > support down the line. > > > >> > And if you find it necessary to use MemoryListener e.g. for performance > > > >> > reasons, then vhost will likely benefit too. > > > >> > > > >> It's technically possible and not hard to do but it prevents > > > >> integrating deeper with core QEMU as the memory API becomes > > > >> thread-safe. > > > >> > > > >> There are two ways to implement dirty logging: > > > >> 1. The vhost log approach which syncs dirty information periodically. > > > >> 2. A cheap thread-safe way to mark dirty outside the global mutex, > > > >> i.e. a thread-safe memory_region_set_dirty(). > > > > > > > > You don't normally want to dirty the whole region, > > > > you want to do this to individual pages. > > > > > > > >> If we can get thread-safe guest memory load/store in QEMU then #2 is > > > >> included. We can switch to using hw/virtio.c instead of > > > >> hw/dataplane/vring.c, we get dirty logging for free, we can drop > > > >> hostmem.c completely, etc. > > > >> > > > >> Stefan > > > > > > > > So why not reuse existing code? If you drop it later it won't > > > > matter what you used ... > > > > > > Let's not lose sight of the forest for the trees here... > > > > > > This whole series is not reusing existing code. That's really the whole > > > point. > > > > > > The point is to take the code (duplication and all) and then do all of > > > the refactoring to use common code in the tree itself. > > > > > > If we want to put this in a hw/staging/ directory, that's fine by me > > > too. > > > > > > Regards, > > > > > > Anthony Liguori > > > > Yes I agree. I think lack of handling for cross regin descriptors > > bothers me a bit more. > > The two things you've mentioned both aren't handled by hw/virtio.c: > > 1. Issue: Indirect descriptors have no alignment restrictions and can > cross regions. > > hw/virtio.c uses vring_desc_flags() and other accessor functions, > which do lduw_phys() - there is no memory region boundary checking > here. Since addresses are aligned this one is fine I think. > 2. Issue: Virtio buffers can cross memory region boundaries. > > hw/virtio.c maps buffers 1:1 using virtqueue_map_sg() and exits if > mapping fails. It does not split buffers if they cross a memory > region. > > These are definitely ugly corner cases but hw/virtio.c is proof that > we're not hitting them in practice. > > Stefan Yes, this one seems ugly. Maybe add a TODO? OK let's assume we want to put it in staging/ I worry about the virtio-blk changes being isolated. Can you put ifdef CONFIG_VIRTIO_BLK_DATA_PLANE around them all to avoid dependency on that header completely if configured out? -- MST