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

[Andy Lutomirski]

On Mon, Dec 14, 2015 at 6:12 AM, Michael S. Tsirkin <mst at 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.  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