From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API Date: Wed, 3 Sep 2014 15:51:11 +0300 Message-ID: <20140903125111.GA14901@redhat.com> References: <1409609814.30640.11.camel@pasglop> <1409691213.30640.37.camel@pasglop> <874mwpkxxi.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: <874mwpkxxi.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 List-Archive: List-Post: To: Rusty Russell Cc: "linux-s390@vger.kernel.org" , Konrad Rzeszutek Wilk , Benjamin Herrenschmidt , Linux Virtualization , Christian Borntraeger , Paolo Bonzini , "linux390@de.ibm.com" , Andy Lutomirski , virtio-dev@lists.oasis-open.org List-ID: On Wed, Sep 03, 2014 at 04:12:01PM +0930, Rusty Russell wrote: > Andy Lutomirski writes: > > There really are virtio devices that are pieces of silicon and not > > figments of a hypervisor's imagination [1]. > > Hi Andy, > > As you're discovering, there's a reason no one has done the DMA > API before. > > So the problem is that ppc64's IOMMU is a platform thing, not a bus > thing. They really do carve out an exception for virtio devices, > because performance (LOTS of performance). It remains to be seen if > other platforms have the same performance issues, but in absence of > other evidence, the answer is yes. > > It's a hack. But having specific virtual-only devices are an even > bigger hack. > > Physical virtio devices have been talked about, but don't actually exist > in Real Life. And someone a virtio PCI card is going to have serious > performance issues: mainly because they'll want the rings in the card's > MMIO region, not allocated by the driver. Why? What's wrong with rings in memory? > Being broken on PPC is really > the least of their problems. > > So, what do we do? It'd be nice if Linux virtio Just Worked under Xen, > though Xen's IOMMU is outside the virtio spec. Since virtio_pci can be > a module, obvious hacks like having xen_arch_setup initialize a dma_ops pointer > exposed by virtio_pci.c is out. Well virtio could probe for xen, it's not a lot of code. > I think the best approach is to have a new feature bit (25 is free), > VIRTIO_F_USE_BUS_MAPPING which indicates that a device really wants to > use the mapping for the bus it is on. A real device would set this, > or it won't work behind an IOMMU. A Xen device would also set this. > > Thoughts? > Rusty. OK and it should then be active even if guest does not ack the feature (so in fact, it would have to be a mandatory feature). That can work, but I still find this a bit inelegant: this is a property of the platform, not of the device. > PS. I cc'd OASIS virtio-dev: it's subscriber only for IP reasons (to > subscribe you have to promise we can use your suggestion in the > standard). Feel free to remove in any replies, but it's part of > the world we live in...