On 17.06.2024 17:17, Demi Marie Obenour wrote: > 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.
And such an ioreq server, which I assume isn't going to run in the Dom0 kernel, will then also need keeping up-to-date on holes in the (virtual) BAR. Oh well ... Jan
