Linux Virtualization - Jul 2015 - [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

On Tue, 2015-07-28 at 10:16 +0200, Paolo Bonzini wrote:
> 
> On 28/07/2015 03:08, Andy Lutomirski wrote:
> > On Mon, Sep 1, 2014 at 10:39 AM, Andy Lutomirski <luto at
amacapital.net> wrote:
> >> This fixes virtio on Xen guests as well as on any other platform
> >> that uses virtio_pci on which physical addresses don't match
bus
> >> addresses.
> >>
> >> This can be tested with:
> >>
> >>     virtme-run --xen xen --kimg arch/x86/boot/bzImage --console
> >>
> >> using virtme from here:
> >>
> >>     https://git.kernel.org/cgit/utils/kernel/virtme/virtme.git
> >>
> >> Without these patches, the guest hangs forever.  With these
patches,
> >> everything works.
> >>
> > 
> > Dusting off an ancient thread.
> > 
> > Now that the dust has accumulated^Wsettled, is it worth pursuing this?
> >  I think the situation is considerably worse than it was when I
> > originally wrote these patches: I think that QEMU now supports a nasty
> > mode in which the guest's PCI bus appears to be behind an IOMMU
but
> > the virtio devices on that bus punch straight through that IOMMU.
> 
> That is an experimental feature (it's x-iommu), so it can change.
> 
> The plan was:
> 
> - for PPC, virtio never honors IOMMU
> 
> - for non-PPC, either have virtio always honor IOMMU, or enforce that
> virtio is not under IOMMU.
> 
I dislike having PPC special cased.

In fact, today x86 guests also assume that virtio bypasses IOMMU I
believe. In fact *all* guests do.

I would much prefer if the information as to whether it honors or not
gets passed to the guest somewhat. My preference goes for passing it via
the virtio config space but there were objections that it should be a
bus property (which is tricky to do with PCI and doesn't properly
reflect the fact that in qemu you can mix & match IOMMU-honoring devices
and bypassing-virtio on the same bus). 

Ben.
> Paolo
> 
> > I have a half-hearted port to modern kernels here:
> > 
> >
https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=virtio_ring_xen
> > 
> > I didn't implement DMA API access for virtio_pci_modern, and I
have no
> > idea what to do about detecting whether a given virtio device honors
> > its IOMMU or not.
> > 
> > --Andy
> >

On 28/07/2015 12:12, Benjamin Herrenschmidt wrote:
>> > That is an experimental feature (it's x-iommu), so it can
change.
>> > 
>> > The plan was:
>> > 
>> > - for PPC, virtio never honors IOMMU
>> > 
>> > - for non-PPC, either have virtio always honor IOMMU, or enforce
that
>> > virtio is not under IOMMU.
>> > 
> I dislike having PPC special cased.
> 
> In fact, today x86 guests also assume that virtio bypasses IOMMU I
> believe. In fact *all* guests do.
This doesn't matter much, since the only guests that implement an IOMMU
in QEMU are (afaik) PPC and x86, and x86 does not yet promise any kind
of stability.
> I would much prefer if the information as to whether it honors or not
> gets passed to the guest somewhat. My preference goes for passing it via
> the virtio config space but there were objections that it should be a
> bus property (which is tricky to do with PCI and doesn't properly
> reflect the fact that in qemu you can mix & match IOMMU-honoring
devices
> and bypassing-virtio on the same bus). 
Yes, for example on x86 it must be passed through the DMAR table.
virtio-pci device must have a separate DRHD for them.  In QEMU, you
could add an "under-iommu" property to PCI bridges, and walk the
hierarchy of bridges to build the DRHDs.

Paolo

On Tue, Jul 28, 2015 at 02:46:20PM +0200, Paolo Bonzini
wrote:
> 
> 
> On 28/07/2015 12:12, Benjamin Herrenschmidt wrote:
> >> > That is an experimental feature (it's x-iommu), so it can
change.
> >> > 
> >> > The plan was:
> >> > 
> >> > - for PPC, virtio never honors IOMMU
> >> > 
> >> > - for non-PPC, either have virtio always honor IOMMU, or
enforce that
> >> > virtio is not under IOMMU.
> >> > 
> > I dislike having PPC special cased.
> > 
> > In fact, today x86 guests also assume that virtio bypasses IOMMU I
> > believe. In fact *all* guests do.
> 
> This doesn't matter much, since the only guests that implement an IOMMU
> in QEMU are (afaik) PPC and x86, and x86 does not yet promise any kind
> of stability.
Hmm I think Jan (cc) said it was already used out there.

> > I would much prefer if the information as to whether it honors or not
> > gets passed to the guest somewhat. My preference goes for passing it
via
> > the virtio config space but there were objections that it should be a
> > bus property (which is tricky to do with PCI and doesn't properly
> > reflect the fact that in qemu you can mix & match IOMMU-honoring
devices
> > and bypassing-virtio on the same bus). 
> 
> Yes, for example on x86 it must be passed through the DMAR table.
> virtio-pci device must have a separate DRHD for them.  In QEMU, you
> could add an "under-iommu" property to PCI bridges, and walk the
> hierarchy of bridges to build the DRHDs.
> 
> Paolo
-- 
MST