Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

Michael S. Tsirkin
2014-09-03T12:51:11Z

On Wed, Sep 03, 2014 at 04:12:01PM +0930, Rusty Russell wrote:
> Andy Lutomirski <luto@amacapital.net> 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...