From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: virtio DMA API? Date: Wed, 27 Aug 2014 13:52:50 +0200 Message-ID: <20140827115249.GA16850@redhat.com> References: <87iole89xg.fsf@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <87iole89xg.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: Benjamin Herrenschmidt , virtualization@lists.linux-foundation.org, Konrad Rzeszutek Wilk , Andy Lutomirski List-Id: virtualization@lists.linuxfoundation.org On Wed, Aug 27, 2014 at 08:40:51PM +0930, Rusty Russell wrote: > Andy Lutomirski writes: > > Currently, a lot of the virtio code assumes that bus (i.e. hypervisor) > > addresses are the same as physical address. This is false on Xen, so > > virtio is completely broken. I wouldn't be surprised if it also > > becomes a problem the first time that someone sticks a physical > > "virtio" device on a 32-bit bus on an ARM SOC with more than 4G RAM. > > > > Would you accept patches to convert virtio_ring and virtio_pci to use > > the DMA APIs? I think that the only real catch will be that > > virtio_ring's approach to freeing indirect blocks is currently > > incompatible with the DMA API -- it assumes that knowing the bus > > address is enough to call kfree, and I don't think that the DMA API > > provides a reverse mapping like that. > > Hi Andy, > > This has long been a source of contention. virtio assumes that > the hypervisor can decode guest-physical addresses. > > PowerPC, in particular, doesn't want to pay the cost of IOMMU > manipulations, and all arguments presented so far for using an IOMMU for > a virtio device are weak. And changing to use DMA APIs would break them > anyway. > > Of course, it's Just A Matter of Code, so it's possible to > create a Xen-specific variant which uses the DMA APIs. I'm not sure > what that would look like in the virtio standard, however. > > Cheers, > Rusty. For x86 as of QEMU 2.0 there's no iommu. So a reasonable thing to do for that platform might be to always use iommu *if it's there*. My understanding is this isn't the case for powerpc? -- MST