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Subject: Re: [virtio-dev] Next VirtIO device for Project Stratos?
On Tue, Sep 06, 2022 at 06:33:36PM +0100, Dr. David Alan Gilbert wrote: > * Stefan Hajnoczi (stefanha@redhat.com) wrote: > > On Sat, Sep 03, 2022 at 07:43:08AM +0000, Alyssa Ross wrote: > > > Hi Alex and everyone else, just catching up on some mail and wanted to > > > clarify some things: > > > > > > Alex Bennée <alex.bennee@linaro.org> writes: > > > > > > > This email is driven by a brain storming session at a recent sprint > > > > where we considered what VirtIO devices we should look at implementing > > > > next. I ended up going through all the assigned device IDs hunting for > > > > missing spec discussion and existing drivers so I'd welcome feedback > > > > from anybody actively using them - especially as my suppositions about > > > > device types I'm not familiar with may be way off! > > > > > > > > [...snip...] > > > > > > > > GPU device / 16 > > > > --------------- > > > > > > > > This is now a fairly mature part of the spec and has implementations is > > > > the kernel, QEMU and a vhost-user backend. However as is commensurate > > > > with the complexity of GPUs there is ongoing development moving from the > > > > VirGL OpenGL encapsulation to a thing called GFXSTREAM which is meant to > > > > make some things easier. > > > > > > > > A potential area of interest here is working out what the differences > > > > are in use cases between virtio-gpu and virtio-wayland. virtio-wayland > > > > is currently a ChromeOS only invention so hasn't seen any upstreaming or > > > > specification work but may make more sense where multiple VMs are > > > > drawing only elements of a final display which is composited by a master > > > > program. For further reading see Alyssa's write-up: > > > > > > > > https://alyssa.is/using-virtio-wl/ > > > > > > > > I'm not sure how widely used the existing vhost-user backend is for > > > > virtio-gpu but it could present an opportunity for a more beefy rust-vmm > > > > backend implementation? > > > > > > As I understand it, virtio-wayland is effectively deprecated in favour > > > of sending Wayland messages over cross-domain virtio-gpu contexts. It's > > > possible to do this now with an upstream kernel, whereas virtio-wayland > > > always required a custom driver in the Chromium kernel. > > > > > > But crosvm is still the only implementation of a virtio-gpu device that > > > supports Wayland over cross-domain contexts, so it would be great to see > > > a more generic implementation. Especially because, while crosvm can > > > share its virtio-gpu device over vhost-user, it does so in a way that's > > > incompatible with the standardised vhost-user-gpu as implemented by > > > QEMU. When I asked the crosvm developers in their Matrix channel what > > > it would take to use the standard vhost-user-gpu variant, they said that > > > the standard variant was lacking functionality they needed, like mapping > > > and unmapping GPU buffers into the guest. > > > > That sounds somewhat similar to virtiofs and its DAX Window, which needs > > vhost-user protocol extensions because of how memory is handled. David > > Gilbert wrote the QEMU virtiofs DAX patches, which are under > > development. > > > > I took a quick look at the virtio-gpu specs. If the crosvm behavior you > > mentioned is covered in the VIRTIO spec then I guess it's the "host > > visible memory region"? > > > > (If it's not in the VIRTIO spec then a spec change needs to be proposed > > first and a vhost-user protocol spec change can then support that new > > virtio-gpu feature.) > > > > The VIRTIO_GPU_CMD_RESOURCE_MAP_BLOB command maps the device's resource > > into the host visible memory region so that the driver can see it. > > > > The virtiofs DAX window uses vhost-user slave channel messages to > > provide file descriptors and offsets for QEMU to mmap. QEMU mmaps the > > file pages into the shared memory region seen by the guest driver. > > > > Maybe an equivalent mechanism is needed for virtio-gpu so a device > > resource file descriptor can be passed to QEMU and then mmapped so the > > guest driver can see the pages? > > > > I think it's possible to unify the virtiofs and virtio-gpu extensions to > > the vhost-user protocol. Two new slave channel messages are needed: "map > > <fd, offset, len> to shared memory resource <n>" and "unmap <offset, > > len> from shared memory resource <n>". Both devices could use these > > messages to implement their respective DAX Window and Blob Resource > > functionality. > > It might be possible; but there's a bunch of lifetime/alignment/etc > questions to be answered. > > For virtiofs DAX we carve out a chunk of a BAR as a 'cache' (unfortunate > name) that we can then do mappings into. > > The VHOST_USER_SLAVE_FS_MAP/UNMAP commands can do the mapping: > https://gitlab.com/virtio-fs/qemu/-/commit/7c29854da484afd7ca95acbd2e4acfc2c75ef491 > https://gitlab.com/virtio-fs/qemu/-/commit/f32bc2524035931856aa218ce18efa029b9eed02 > > those might do what you want if you can figure out a way to generalise > the bar to map them into. > > There are some problems; KVM gets really really upset if you try and > access an area that doesn't have a mapping or is mapped to a truncated > file; do you want the guest to be able to crash like that? I think you are pointing out the existing problems with virtiofs map/unmap and not new issues related to virtio-gpu or generalizing the vhost-user messages? There are a few possibilities for dealing with unmapped ranges in Shared Memory Regions: 1. Reserve the unused Shared Memory Region ranges with mmap(PROT_NONE) so that accesses to unmapped pages result in faults. 2. Map zero pages that are either: a. read-only b. read-write but discard stores c. private/anonymous memory virtiofs does #1 and has trouble with accesses to unmapped areas because KVM's MMIO dispatch loop gets upset. On top of that virtiofs also needs a way to inject the fault into the guest so that the truncated mmap case can be detected in the guest. The situation is probably easier for virtio-gpu than for virtiofs. I think the underlying host files won't be truncated and guest userspace processes cannot access unmapped pages. So virtio-gpu is less susceptible to unmapped accesses. But we still need to implement unmapped access semantics. I don't know enough about CPU memory to suggest a solution for injecting unmapped access faults. Maybe you can find someone who can help. I wonder if pmem or CXL devices have similar requirements? Stefan
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