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

[Andy Lutomirski]

On Wed, Aug 27, 2014 at 9:13 AM, Christopher Covington
<cov at codeaurora.org> wrote:
> On 08/27/2014 11:50 AM, Andy Lutomirski wrote:
>> On Wed, Aug 27, 2014 at 8:45 AM, Michael S. Tsirkin <mst at 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 at gmail.com> wrote:
>>>>>
>>>>> On Tue, Aug 26, 2014 at 10:16 PM, Andy Lutomirski <luto at 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
>

Since I know nothing about LPAE, I'll leave this to you :)  But I'll
cc you on patch v2 soon, and please tell me whether my code makes
sense on ARM.

(My attempt to boot-test s390 failed because I have no clue what
command-line options to pass to QEMU.  If anyone wants to give me some
pointers to get a working configuration with -kernel and some kind of
console, I can add support to virtme.  Alas, I think that no one ever
bothered to implement 9p over virtio-ccw in QEMU.  Why exactly does
the virtio stuff in QEMU require that you instantiate virtio-9p-pci
instead of just asking for an appropriate virtio dievice?)

--Andy