Re: [PATCH 0/3] virtio: Clean up scatterlists and use the DMA API

[Christopher Covington <cov@codeaurora.org>]

On 08/27/2014 11:50 AM, Andy Lutomirski wrote:
> On Wed, Aug 27, 2014 at 8:45 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
>> On Wed, Aug 27, 2014 at 08:11:15AM -0700, Andy Lutomirski wrote:
>>> On Aug 26, 2014 11:46 PM, "Stefan Hajnoczi" <stefanha@gmail.com> wrote:
>>>>
>>>> On Tue, Aug 26, 2014 at 10:16 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>>>> There are two outstanding issues.  virtio_net warns if DMA debugging
>>>>> is on because it does DMA from the stack.  (The warning is correct.)
>>>>> This also is likely to do something unpleasant to s390.
>>>>> (Maintainers are cc'd -- I don't know what to do about it.)
>>>>
>>>> This changes the semantics of vring and breaks existing guests when
>>>> bus address != physical address.
>>>>
>>>> Can you use a transport feature bit to indicate that bus addresses are
>>>> used?  That way both approaches can be supported.
>>>
>>> I can try to support both styles of addressing, but I don't think that
>>> this can be negotiated between the device (i.e. host or physical
>>> virtio-speaking device) and the guest.  In the Xen case that I care
>>> about (Linux on Xen on KVM), the host doesn't know about the
>>> translation at all -- Xen is an intermediate layer that only the guest
>>> is aware of.  In this case, there are effectively two layers of
>>> virtualization, and only the inner one (Xen) knows about the
>>> translation despite the fact that the the outer layer is the one
>>> providing the virtio device.
>>>
>>> I could change virtio_ring to use the DMA API only if requested by the
>>> lower driver (virtio_pci, etc) and to have only virtio_pci enable that
>>> feature.  Will that work for all cases?
>>>
>>> On s390, this shouldn't work just like the current code.  On x86, I
>>> think that if QEMU ever starts exposing an IOMMU attached to a
>>> virtio-pci device, then QEMU should expect that IOMMU to be used.  If
>>> QEMU expects to see physical addresses, then it shouldn't advertise an
>>> IOMMU.  Since QEMU doesn't currently support guest IOMMUs, this should
>>> be fine for everything that uses QEMU.
>>>
>>> At least x86's implementation of the DMA ops for devices that aren't
>>> behind an IOMMU should be very fast.
>>>
>>> Are there any other weird cases for which this might be a problem?
>>>
>>>>
>>>> Please also update the virtio specification:
>>>> https://tools.oasis-open.org/version-control/browse/wsvn/virtio/
>>>>
>>>
>>> I'm not sure it will need an update.  Perhaps a note in the PCI
>>> section indicating that, if the host expects the guest to program an
>>> IOMMU, then it should use the appropriate platform-specific mechanism
>>> to expose that IOMMU.
>>>
>>> --Andy
>>
>> If there's no virtio mechanism to negotate enabling/disabling
>> translations, then specification does not need an extension.
> 
> It wouldn't shock me if someone wants to negotiate this for
> virtio_mmio some day, but that might be more of a device tree thing.
> I have no idea how that works, but I think it can wait until someone
> wants it.

At one point I wanted VirtIO-MMIO to not fail miserably on ARM LPAE systems.

http://lists.infradead.org/pipermail/linux-arm-kernel/2014-March/241613.html

Christopher

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