Subject: Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API
"Michael S. Tsirkin" <firstname.lastname@example.org> writes: > On Wed, Sep 03, 2014 at 04:12:01PM +0930, Rusty Russell wrote: >> Andy Lutomirski <email@example.com> writes: >> > There really are virtio devices that are pieces of silicon and not >> > figments of a hypervisor's imagination . >> >> 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? AFAICT, the card would have to access guest memory to read it, using multiple DMA cycles. That's going to be slow. >> 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. We could, but I think this is going to be a more general problem in future. x86 is heading down the IOMMU path, and they're likely to suffer similarly. >> 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. True. If a device needs it though, we're no worse of having a device which doesn't work if the driver understand the feature than we were before. Cheers, Rusty.