Re: Design session notes: GPU acceleration in Xen

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Mon, Jun 17, 2024 at 11:07:54AM +0200, Jan Beulich wrote:
> On 14.06.2024 18:44, Demi Marie Obenour wrote:
> > On Fri, Jun 14, 2024 at 10:12:40AM +0200, Jan Beulich wrote:
> >> On 14.06.2024 09:21, Roger Pau Monné wrote:
> >>> I'm not sure it's possible to ensure that when using system RAM such
> >>> memory comes from the guest rather than the host, as it would likely
> >>> require some very intrusive hooks into the kernel logic, and
> >>> negotiation with the guest to allocate the requested amount of
> >>> memory and hand it over to dom0.  If the maximum size of the buffer is
> >>> known in advance maybe dom0 can negotiate with the guest to allocate
> >>> such a region and grant it access to dom0 at driver attachment time.
> >>
> >> Besides the thought of transiently converting RAM to kind-of-MMIO, this
> >> makes me think of another possible option: Could Dom0 transfer ownership
> >> of the RAM that wants mapping in the guest (remotely resembling
> >> grant-transfer)? Would require the guest to have ballooned down enough
> >> first, of course. (In both cases it would certainly need working out how
> >> the conversion / transfer back could be made work safely and reasonably
> >> cleanly.)
> > 
> > The kernel driver needs to be able to reclaim the memory at any time.
> > My understanding is that this is used to migrate memory between VRAM and
> > system RAM.  It might also be used for other purposes.
> 
> Except: How would the kernel driver reclaim the memory when it's mapped
> by a DomU?

The Xen driver in dom0 will register for MMU notifier callbacks.  When
the kernel driver reclaims the memory, the Xen driver will be notified,
and it will issue a hypercall that tells Xen to remove the memory from
the DomU's address space.  Subsequent accesses to the pages will trigger
a stage 2 translation fault that is handled by an IOREQ server.

For I/O memory, this is already possible via XEN_DOMCTL_memory_mapping.
The proposal in this thread is to make this possible for system RAM as
well.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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