On 17.06.2024 02:38, Demi Marie Obenour wrote: > On Fri, Jun 14, 2024 at 10:39:37AM +0200, Roger Pau Monné wrote: >> On Fri, Jun 14, 2024 at 10:12:40AM +0200, Jan Beulich wrote: >>> On 14.06.2024 09:21, Roger Pau Monné wrote: >>>> On Fri, Jun 14, 2024 at 08:38:51AM +0200, Jan Beulich wrote: >>>>> On 13.06.2024 20:43, Demi Marie Obenour wrote: >>>>>> GPU acceleration requires that pageable host memory be able to be mapped >>>>>> into a guest. >>>>> >>>>> I'm sure it was explained in the session, which sadly I couldn't attend. >>>>> I've been asking Ray and Xenia the same before, but I'm afraid it still >>>>> hasn't become clear to me why this is a _requirement_. After all that's >>>>> against what we're doing elsewhere (i.e. so far it has always been >>>>> guest memory that's mapped in the host). I can appreciate that it might >>>>> be more difficult to implement, but avoiding to violate this fundamental >>>>> (kind of) rule might be worth the price (and would avoid other >>>>> complexities, of which there may be lurking more than what you enumerate >>>>> below). >>>> >>>> My limited understanding (please someone correct me if wrong) is that >>>> the GPU buffer (or context I think it's also called?) is always >>>> allocated from dom0 (the owner of the GPU). The underling memory >>>> addresses of such buffer needs to be mapped into the guest. The >>>> buffer backing memory might be GPU MMIO from the device BAR(s) or >>>> system RAM, and such buffer can be paged by the dom0 kernel at any >>>> time (iow: changing the backing memory from MMIO to RAM or vice >>>> versa). Also, the buffer must be contiguous in physical address >>>> space. >>> >>> This last one in particular would of course be a severe restriction. >>> Yet: There's an IOMMU involved, isn't there? >> >> Yup, IIRC that's why Ray said it was much more easier for them to >> support VirtIO GPUs from a PVH dom0 rather than classic PV one. >> >> It might be easier to implement from a classic PV dom0 if there's >> pv-iommu support, so that dom0 can create it's own contiguous memory >> buffers from the device PoV. > > What makes PVH an improvement here? I thought PV dom0 uses an identity > mapping for the IOMMU,
True, but see how Roger mentioned PV IOMMU (which would allow a domain to move away from this identity mapping). Jan > while a PVH dom0 uses an IOMMU that mirrors the > dom0 second-stage page tables. In both cases, the device physical > addresses are identical to dom0’s physical addresses. > > PV is terrible for many reasons, so I’m okay with focusing on PVH dom0, > but I’d like to know why there is a difference.
