Re: [PATCH RFC] fixup! virtio: convert to use DMA api

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Apr 19, 2016 at 1:54 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Tue, Apr 19, 2016 at 01:27:29PM -0700, Andy Lutomirski wrote:
>> On Tue, Apr 19, 2016 at 1:16 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
>> > On Tue, Apr 19, 2016 at 11:01:38AM -0700, Andy Lutomirski wrote:
>> >> On Tue, Apr 19, 2016 at 10:49 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
>> >> > On Tue, Apr 19, 2016 at 12:26:44PM -0400, David Woodhouse wrote:
>> >> >> On Tue, 2016-04-19 at 19:20 +0300, Michael S. Tsirkin wrote:
>> >> >> >
>> >> >> > > I thought that PLATFORM served that purpose.  Woudn't the host
>> >> >> > > advertise PLATFORM support and, if the guest doesn't ack it, the host
>> >> >> > > device would skip translation?  Or is that problematic for vfio?
>> >> >> >
>> >> >> > Exactly that's problematic for security.
>> >> >> > You can't allow guest driver to decide whether device skips security.
>> >> >>
>> >> >> Right. Because fundamentally, this *isn't* a property of the endpoint
>> >> >> device, and doesn't live in virtio itself.
>> >> >>
>> >> >> It's a property of the platform IOMMU, and lives there.
>> >> >
>> >> > It's a property of the hypervisor virtio implementation, and lives there.
>> >>
>> >> It is now, but QEMU could, in principle, change the way it thinks
>> >> about it so that virtio devices would use the QEMU DMA API but ask
>> >> QEMU to pass everything through 1:1.  This would be entirely invisible
>> >> to guests but would make it be a property of the IOMMU implementation.
>> >> At that point, maybe QEMU could find a (platform dependent) way to
>> >> tell the guest what's going on.
>> >>
>> >> FWIW, as far as I can tell, PPC and SPARC really could, in principle,
>> >> set up 1:1 mappings in the guest so that the virtio devices would work
>> >> regardless of whether QEMU is ignoring the IOMMU or not -- I think the
>> >> only obstacle is that the PPC and SPARC 1:1 mappings are currectly set
>> >> up with an offset.  I don't know too much about those platforms, but
>> >> presumably the layout could be changed so that 1:1 really was 1:1.
>> >>
>> >> --Andy
>> >
>> > Sure. Do you see any reason why the decision to do this can't be
>> > keyed off the virtio feature bit?
>>
>> I can think of three types of virtio host:
>>
>> a) virtio always bypasses the IOMMU.
>>
>> b) virtio never bypasses the IOMMU (unless DMAR tables or similar say
>> it does) -- i.e. virtio works like any other device.
>>
>> c) virtio may bypass the IOMMU depending on what the guest asks it to do.
>
> d) some virtio devices bypass the IOMMU and some don't,
> e.g. it's harder to support IOMMU with vhost.
>
>
>> If this is keyed off a virtio feature bit and anyone tries to
>> implement (c), the vfio is going to have a problem.  And, if it's
>> keyed off a virtio feature bit, then (a) won't work on Xen or similar
>> setups unless the Xen hypervisor adds a giant and probably unreliable
>> kludge to support it.  Meanwhile, 4.6-rc works fine under Xen on a
>> default x86 QEMU configuration, and I'd really like to keep it that
>> way.
>>
>> What could plausibly work using a virtio feature bit is for a device
>> to say "hey, I'm a new device and I support the platform-defined IOMMU
>> mechanism".  This bit would be *set* on default IOMMU-less QEMU
>> configurations and on physical virtio PCI cards.
>
> And clear on xen.

How?  QEMU has no idea that the guest is running Xen.