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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Tue, Dec 15, 2015 at 12:40 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> 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@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.
>
> 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.

I'd be in favor of that approach, except that apparently PowerPC can't
do it (the 1:1 mappings have an offset).  I *think* that x86 can do
it.

I'll re-send the series with DMA API defaulted off except on Xen once
the merge window closes.

--Andy

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