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Re: [Xen-devel] [PATCH RFC 0/3] Xen on Virtio

On Mon, 14 Dec 2015, Andy Lutomirski wrote:
> On Mon, Dec 14, 2015 at 6:12 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > On Mon, Dec 14, 2015 at 02:00:05PM +0000, David Vrabel wrote:
> >> On 07/12/15 16:19, Stefano Stabellini wrote:
> >> > Hi all,
> >> >
> >> > this patch series introduces support for running Linux on top of Xen
> >> > inside a virtual machine with virtio devices (nested virt scenario).
> >> > The problem is that Linux virtio drivers use virt_to_phys to get the
> >> > guest pseudo-physical addresses to pass to the backend, which doesn't
> >> > work as expected on Xen.
> >> >
> >> > Switching the virtio drivers to the dma APIs (dma_alloc_coherent,
> >> > dma_map/unmap_single and dma_map/unmap_sg) would solve the problem, as
> >> > Xen support in Linux provides an implementation of the dma API which
> >> > takes care of the additional address conversions. However using the dma
> >> > API would increase the complexity of the non-Xen case too. We would also
> >> > need to keep track of the physical or virtual address in addition to the
> >> > dma address for each vring_desc to be able to free the memory in
> >> > detach_buf (see patch #3).
> >> >
> >> > Instead this series adds few obvious checks to perform address
> >> > translations in a couple of key places, without changing non-Xen code
> >> > paths. You are welcome to suggest improvements or alternative
> >> > implementations.
> >>
> >> Andy Lutomirski also looked at this.  Andy what happened to this work?
> >>
> >> David
> >
> > The approach there was to try and convert all virtio to use DMA
> > API unconditionally.
> > This is reasonable if there's a way for devices to request
> > 1:1 mappings individually.
> > As that is currently missing, that patchset can not be merged yet.
> >
> I still don't understand why *devices* need the ability to request
> anything in particular.  In current kernels, devices that don't have
> an iommu work (and there's no choice about 1:1 or otherwise) and
> devices that have an iommu fail spectacularly.  With the patches,
> devices that don't have an iommu continue to work as long as the DMA
> API and/or virtio correctly knows that there's no iommu.  Devices that
> do have an iommu work fine, albeit slower than would be ideal.  In my
> book, slower than would be ideal is strictly better than crashing.
> The real issue is *detecting* whether there's an iommu, and the string
> of bugs in that area (buggy QEMU for the Q35 thing and complete lack
> of a solution for PPC and SPARC is indeed a problem).
> I think that we could apply the series ending here:
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=virtio_dma&id=ad9d43052da44ce18363c02ea597dde01eeee11b
> and the only regression (performance or functionality) would be that
> the buggy Q35 iommu configuration would stop working until someone
> fixed it in QEMU.  That should be okay -- it's explicitly
> experimental.  (Xen works with that series applied.)   (Actually,
> there might be a slight performance regression on PPC due to extra
> unused mappings being created.  It would be straightforward to hack
> around that in one of several ways.)
> Am I missing something?

Your changes look plausible and if they fix Xen on virtio I am happy
with them.  I didn't choose the DMA API approach because, although it
looks cleaner, I acknowledge that is a bit invasive.

I suggest that the virtio maintainers consider one of the two approaches
for inclusion because they fix a real issue.

If you would rather avoid the DMA API, then I would be happy to work
with you to evolve my current series in a direction of your liking.
Please advise on how to proceed.

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