Gurchetan Singh <gurchetansingh@xxxxxxxxxxxx> writes: > On Thu, Aug 8, 2024 at 3:38 AM Sergio Lopez Pascual <slp@xxxxxxxxxx> wrote: > >> Gurchetan Singh <gurchetansingh@xxxxxxxxxxxx> writes: >> >> > On Tue, Aug 6, 2024 at 1:15 PM Rob Clark <robdclark@xxxxxxxxx> wrote: >> > >> >> On Tue, Aug 6, 2024 at 9:15 AM Gurchetan Singh >> >> <gurchetansingh@xxxxxxxxxxxx> wrote: >> >> > >> >> > >> >> > >> >> > On Mon, Aug 5, 2024 at 2:14 AM Sergio Lopez Pascual <slp@xxxxxxxxxx> >> >> wrote: >> >> >> >> >> >> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> writes: >> >> >> >> >> >> > On 7/23/24 14:49, Sergio Lopez wrote: >> >> >> >> There's an incresing number of machines supporting multiple page >> >> sizes >> >> >> >> and on these machines the host and a guest can be running, each >> one, >> >> >> >> with a different page size. >> >> >> >> >> >> >> >> For what pertains to virtio-gpu, this is not a problem if the page >> >> size >> >> >> >> of the guest happens to be bigger or equal than the host, but will >> >> >> >> potentially lead to failures in memory allocations and/or mappings >> >> >> >> otherwise. >> >> >> > >> >> >> > Please describe concrete problem you're trying to solve. Guest >> memory >> >> >> > allocation consists of guest pages, I don't see how knowledge of >> host >> >> >> > page size helps anything in userspace. >> >> >> > >> >> >> > I suspect you want this for host blobs, but then it should be >> >> >> > virtio_gpu_vram_create() that should use max(host_page_sz, >> >> >> > guest_page_size), AFAICT. It's kernel who is responsible for memory >> >> >> > management, userspace can't be trusted for doing that. >> >> >> >> >> >> Mesa's Vulkan/Venus uses CREATE_BLOB to request the host the creation >> >> >> and mapping into the guest of device-backed memory and shmem regions. >> >> >> The CREATE_BLOB ioctl doesn't update >> drm_virtgpu_resource_create->size, >> >> >> so the guest kernel (and, as a consequence, the host kernel) can't >> >> >> override the user's request. >> >> >> >> >> >> I'd like Mesa's Vulkan/Venus in the guest to be able to obtain the >> host >> >> >> page size to align the size of the CREATE_BLOB requests as required. >> >> > >> >> > >> >> > gfxstream solves this problem by putting the relevant information in >> the >> >> capabilities obtained from the host: >> >> > >> >> > >> >> >> https://android.googlesource.com/platform/hardware/google/gfxstream/+/refs/heads/main/host/virtio-gpu-gfxstream-renderer.cpp#1691 >> >> > >> >> > If you want to be paranoid, you can also validate the >> >> ResourceCreateBlob::size is properly host-page aligned when that request >> >> reaches the host. >> >> > >> >> > So you can probably solve this problem using current interfaces. >> >> Whether it's cleaner for all context types to use the capabilities, or >> have >> >> all VMMs to expose VIRTIO_GPU_F_HOST_PAGE_SIZE, would be the >> cost/benefit >> >> tradeoff. >> >> > >> >> >> >> I guess solving it in a context-type specific way is possible. But I >> >> think it is a relatively universal constraint. And maybe it makes >> >> sense for virtgpu guest kernel to enforce alignment (at least it can >> >> return an error synchronously) in addition to the host. >> >> >> > >> > virtio-media may have support for VIRTIO_MEDIA_CMD_MMAP too, so could run >> > into this issue. >> > >> > >> https://github.com/chromeos/virtio-media?tab=readme-ov-file#shared-memory-regions >> > >> > virtio-fs also has the DAX window which uses the same memory mapping >> > mechanism. >> > >> > https://virtio-fs.gitlab.io/design.html >> > >> > Maybe this should not be a virtio-gpu thing, but a virtio thing? >> >> This is true, but finding a common place to put the page size is really >> hard in practice. I don't think we can borrow space in the feature bits >> for that (and that would probably be abusing its purpose quite a bit) >> and extending the transport configuration registers is quite cumbersome >> and, in general, undesirable. >> >> That leaves us with the device-specific config space, and that implies a >> device-specific feature bit as it's implemented in this series. >> >> The Shared Memory Regions on the VIRTIO spec, while doesn't talk >> specifically about page size, also gives us a hint about this being the >> right direction: >> > > Can't we just modify the Shared Memory region PCI capability to include > page size? We can either: > > 1) keep the same size struct + header (VIRTIO_PCI_CAP_SHARED_MEMORY_CFG), > and just hijack one of the padding fields. If the padding field is zero, we > can just say it's 4096. Yes, we can turn that padding into "__le16 page_size_order" to store "PAGE_SIZE >> 12". That should be enough to secure some future-proofing. There's also some space in the "MMIO Device Register Layout" to store it as a 16 bit or 32 bit value. This would require proposing it as a change to the VIRTIO specs. Do you want to do it yourself or should I take the initiative? Thanks, Sergio.
