From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:40069) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RFymr-00012K-BY for qemu-devel@nongnu.org; Mon, 17 Oct 2011 21:48:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1RFymq-0002By-BV for qemu-devel@nongnu.org; Mon, 17 Oct 2011 21:48:13 -0400 Received: from e23smtp08.au.ibm.com ([202.81.31.141]:42399) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RFymp-0001cT-NJ for qemu-devel@nongnu.org; Mon, 17 Oct 2011 21:48:12 -0400 Received: from /spool/local by au.ibm.com with XMail ESMTP for from ; Tue, 18 Oct 2011 01:46:26 +1000 Received: from d23av03.au.ibm.com (d23av03.au.ibm.com [9.190.234.97]) by d23relay05.au.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id p9I1j3Tf1900702 for ; Tue, 18 Oct 2011 12:45:07 +1100 Received: from d23av03.au.ibm.com (loopback [127.0.0.1]) by d23av03.au.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id p9I1lY2t000766 for ; Tue, 18 Oct 2011 12:47:35 +1100 Date: Tue, 18 Oct 2011 12:46:50 +1100 From: David Gibson Message-ID: <20111018014650.GB6655@truffala.fritz.box> References: <20111002102547.GC30747@redhat.com> <4E883CF4.6060606@redhat.com> <20111002105238.GE30747@redhat.com> <4E8843DB.1020404@redhat.com> <20111002111700.GF30747@redhat.com> <4E885286.30905@redhat.com> <20111002121426.GK30747@redhat.com> <4E89B5D1.4080600@us.ibm.com> <20111014021407.GB4580@truffala.fritz.box> <4E9AD909.1000509@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4E9AD909.1000509@redhat.com> Subject: Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Avi Kivity Cc: Anthony Liguori , "Michael S. Tsirkin" , joerg.roedel@amd.com, Paul 'Rusty' Russell , qemu-devel@nongnu.org, agraf@suse.de, kraxel@redhat.com, eduard.munteanu@linux360.ro, rth@twiddle.net On Sun, Oct 16, 2011 at 03:15:53PM +0200, Avi Kivity wrote: > On 10/14/2011 04:14 AM, David Gibson wrote: > > > Virtio is a very, very special case. virtio requires coherent RAM access. > > > > Right. Virtio's access to memory is *not* emulated PCI DMA, it's > > god-like hypervisor access to guest system memory. It should > > correctly bypass any IOMMU, and so should remain as > > cpu_physical_memory_rw() or the atomic accessors, rather than being > > converted to this new API. > > virtio should definitely not bypass an iommu. So, I just had a chat with Rusty about this. Perhaps it shouldn't, but it does. The spec is in terms of guest physical addresses, not bus/DMA addresses, and more to the point the Linux driver does *not* do the necessary dma_map() and unmap operations to treat this as a PCI DMA. So like it or not, god-like hypervisor access rather than emulated PCI DMA is what it does. > A guest may assign a > virtio device to nested guests, and would wish it confined by the > emulated iommu. Well, that would be nice, but it can't be done. It could be fixed, but it would be an incompatible change so it would need a new feature bit corresponding changes in the Linux driver to do the dma map/unmap if it accepts the "respect IOMMU" feature. > More generally, a guest sees a virtio device as just another pci device, > and has no way to tell that it bypasses the iommu. Well, except the fact that the driver knows its a virtio device, because it's a virtio driver. It's not like you can write a driver that uses PCI DMA without knowing the particulars of the device you're using. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson