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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Dec 14, 2015 at 10:27:52AM -0800, Andy Lutomirski wrote:
> On Mon, Dec 14, 2015 at 6:12 AM, Michael S. Tsirkin <mst@redhat.com> 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.

See below.

> 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?
> 
> --Andy

I think there's more to virtio than just QEMU.

I have no idea whether anyone implemented hypervisors with an IOMMU.
virtio bypassing iommu makes a lot of sense so it did this since
forever. I do not feel comfortable changing guest/hypervisor ABI and
waiting for people to complain.

But we do want to fix Xen.

Let's do this slowly, and whitelist the configurations that
require DMA API to work, so we know we are not breaking anything.

For example, test a device flag and use iommu if set.
Currently, set it if xen_pv_domain is enabled.
We'll add more as more platforms gain IOMMU support
for virtio and we find ways to identify them.

It would be kind of a mix of what you did and what Stefano did.

And alternative would be a quirk: make DMA API create 1:1 mappings for
virtio devices only.  Then teach Xen pv to ignore this quirk.  This is
what I referred to above.
For example, something like DMA_ATTR_IOMMU_BYPASS would do the trick
nicely. If there's a chance that's going to be upstream, we
could use that.

-- 
MST