Design session notes: GPU acceleration in Xen

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

GPU acceleration requires that pageable host memory be able to be mapped
into a guest.  This requires changes to all of the Xen hypervisor, Linux
kernel, and userspace device model.

### Goals

 - Allow any userspace pages to be mapped into a guest.
 - Support deprivileged operation: this API must not be usable for privilege 
escalation.
 - Use MMU notifiers to ensure safety with respect to use-after-free.

### Hypervisor changes

There are at least two Xen changes required:

1. Add a new flag to IOREQ that means "retry this instruction".

   An IOREQ server can set this flag after having successfully handled a
   page fault.  It is expected that the IOREQ server has successfully
   mapped a page into the guest at the location of the fault.
   Otherwise, the same fault will likely happen again.

2. Add support for `XEN_DOMCTL_memory_mapping` to use system RAM, not
   just IOMEM.  Mappings made with `XEN_DOMCTL_memory_mapping` are
   guaranteed to be able to be successfully revoked with
   `XEN_DOMCTL_memory_mapping`, so all operations that would create
   extra references to the mapped memory must be forbidden.  These
   include, but may not be limited to:

   1. Granting the pages to the same or other domains.
   2. Mapping into another domain using `XEN_DOMCTL_memory_mapping`.
   3. Another domain accessing the pages using the foreign memory APIs,
      unless it is privileged over the domain that owns the pages.

   Open question: what if the other domain goes away?  Ideally,
   unmapping would (vacuously) succeed in this case.  Qubes OS doesn't
   care about domid reuse but others might.

### Kernel changes

Linux will add support for mapping userspace memory into an emulated PCI
BAR.  This requires Linux to automatically revoke access when needed.

There will be an IOREQ server that handles page faults.  The discussion
assumed that this handling will happen in kernel mode, but if handling
in user mode is simpler that is also an option.

There is no async #PF in Xen (yet), so the entire vCPU will be blocked
while the fault is handled.  This is not great for performance, but
correctness comes first.

There will be a new kernel ioctl to perform the mapping.  A possible C
prototype (presented at design session, but not discussed there):

    struct xen_linux_register_memory {
        uint64_t pointer;
        uint64_t size;
        uint64_t gpa;
        uint32_t id;
        uint32_t guest_domid;
    };
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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