Re: Why memory lending is needed for GPU acceleration

Jan Beulich
2026-03-30T10:25:30Z

On 30.03.2026 12:15, Teddy Astie wrote:
> Le 29/03/2026 à 19:32, Demi Marie Obenour a écrit :

May I ask that the two of you please properly separate To: vs Cc:?

Thanks, Jan

>> On 3/24/26 10:17, Demi Marie Obenour wrote:
>>> Here is a proposed design document for supporting mapping GPU VRAM
>>> and/or file-backed memory into other domains. It's not in the form of
>>> a patch because the leading + characters would just make it harder to
>>> read for no particular gain, and because this is still RFC right now.
>>> Once it is ready to merge, I'll send a proper patch. Nevertheless,
>>> you can consider this to be
>>>
>>> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
>>>
>>> This approach is very different from the "frontend-allocates"
>>> approach used elsewhere in Xen. It is very much Linux-centric,
>>> rather than Xen-centric. In fact, MMU notifiers were invented for
>>> KVM, and this approach is exactly the same as the one KVM implements.
>>> However, to the best of my understanding, the design described here is
>>> the only viable one. Linux MM and GPU drivers require it, and changes
>>> to either to relax this requirement will not be accepted upstream.
>>
>> Teddy Astie (CCd) proposed a couple of alternatives on Matrix:
>>
>> 1. Create dma-bufs for guest pages and import them into the host.
>>
>> This is a win not only for Xen, but also for KVM. Right now, shared
>> (CPU) memory buffers must be copied from the guest to the host,
>> which is pointless. So fixing that is a good thing! That said,
>> I'm still concerned about triggering GPU driver code-paths that
>> are not tested on bare metal.
>>
>> 2. Use PASID and 2-stage translation so that the GPU can operate in
>> guest physical memory.
>>
>> This is also a win. AMD XDNA absolutely requires PASID support,
>> and apparently AMD GPUs can also use PASID. So being able to use
>> PASID is certainly helpful.
>>
>> However, I don't think either approach is sufficient for two reasons.
>>
>> First, discrete GPUs have dedicated VRAM, which Xen knows nothing about.
>> Only dom0's GPU drivers can manage VRAM, and they will insist on being
>> able to migrate it between the CPU and the GPU. Furthermore, VRAM
>> can only be allocated using GPU driver ioctls, which will allocate
>> it from dom0-owned memory.
>>
>> Second, Certain Wayland protocols, such as screencapture, require programs
>> to be able to import dmabufs. Both of the above solutions would
>> require that the pages be pinned. I don't think this is an option,
>> as IIUC pin_user_pages() fails on mappings of these dmabufs. It's why
>> direct I/O to dmabufs doesn't work.
>>
>
> I suppose it fails because of the RAM/VRAM constraint you said
> previously. If the location of the memory stays the same (i.e guest
> memory mapping), pin should be almost "no-op".
>
> (though, having dma-buf buffers coming from GPU drivers failing to pin
> is probably not a good thing in term of stability; some stuff like
> cameras probably break as a result; but I'm not a expert on that subject)
>
>> To the best of my knowledge, these problems mean that lending memory
>> is the only way to get robust GPU acceleration for both graphics and
>> compute workloads under Xen. Simpler approaches might work for pure
>> compute workloads, for iGPUs, or for drivers that have Xen-specific
>> changes. None of them, however, support graphics workloads on dGPUs
>> while using the GPU driver the same way bare metal workloads do.
>>
>> Linux's graphics stack is massive, and trying to adapt it to work with
>> Xen isn't going to be sustainable in the long term. Adapting Xen to
>> fit the graphics stack is probably more work up front, but it has the
>> advantage of working with all GPU drivers, including ones that have not
>> been written yet. It also means that the testing done on bare metal is
>> still applicable, and that bugs found when using this driver can either
>> be reproduced on bare metal or can be fixed without driver changes.
>
> One of my main concerns was about whether dma-buf can be used as
> "general purpose" GPU buffers; what I read in driver code suggest it
> should be fine, but it's a bit on the edge.
>
>>
>> Finally, I'm not actually attached to memory lending at all. It's a
>> lot of complexity, and it's not at all similar to how the rest of
>> Xen works. If someone else can come up with a better solution that
>> doesn't require GPU driver changes, I'd be all for it. Unfortunately,
>> I suspect none exists. One can make almost anything work if one is
>> willing to patch the drivers, but I am virtually certain that this
>> will not be long-term sustainable.
>>
>
> There's also the virtio-gpu side to consider. Blob mechanism appears to
> insist that GPU memory to come from the host by allowing buffers that
> aren't bound to virtio-gpu BAR yet (that also complexifies the KVM
> situation).
>
> You can have GPU memory that exists in virtio-gpu, without being
> guest-visible, then the guest can map it on its own BAR.
>
>> If Xen had its own GPU drivers, the situation would be totally
>> different. However, Xen must rely on Linux's GPU drivers, and that
>> means it must play by their rules.
>
>
>
>
> --
> Teddy Astie | Vates XCP-ng Developer
>
> XCP-ng & Xen Orchestra - Vates solutions
>
> web: https://vates.tech
>
>