On Wed, Jan 31, 2024 at 4:26 PM James Houghton <jthoughton@xxxxxxxxxx> wrote: > > On that note, I think we need to drop the patch that causes > read-faults in RO memslots to go through fast GUP. KVM didn't do that > for a good reason[1]. Thanks a ton for catching this James! That's some informative reading, and a good reminder to be extra careful messing with stuff I don't quite understand. On Thu, Feb 1, 2024 at 8:28 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Thu, Feb 01, 2024, Oliver Upton wrote: > > On Wed, Jan 31, 2024 at 04:26:08PM -0800, James Houghton wrote: > > > On Wed, Jan 31, 2024 at 2:00 PM Anish Moorthy <amoorthy@xxxxxxxxxx> wrote: > > > > [...] > > > > > On that note, I think we need to drop the patch that causes > > > read-faults in RO memslots to go through fast GUP. KVM didn't do that > > > for a good reason[1]. > > > > > > That would break KVM_EXIT_ON_MISSING for RO memslots, so I think that > > > the right way to implement KVM_EXIT_ON_MISSING is to have > > > hva_to_pfn_slow pass FOLL_NOFAULT, at least for the RO memslot case. > > > We still get the win we're looking for: don't grab the userfaultfd > > > locks. > > > > Is there even a legitimate use case that warrants optimizing faults on > > RO memslots? My expectation is that the VMM uses these to back things > > like guest ROM, or at least that's what QEMU does. In that case I'd > > expect faults to be exceedingly rare, and if the VMM actually cared it > > could just pre-populate the primary mapping. > > > > I understand why we would want to support KVM_EXIT_ON_MISSING for the > > sake of completeness of the UAPI, but it'd be surprising if this > > mattered in the context of post-copy. > > Yeah, I let's just make KVM_EXIT_ON_MISSING mutually exclusive with > KVM_MEM_READONLY. KVM (oviously) honors the primary MMU protections, so userspace > can (and does) map read-only memory into the guest without READONLY. As Oliver > pointed out, making the *memslot* RO is intended for use cases where userspace > wants writes to be treated like emulated MMIO. > > We can always add support in the future in the extremely unlikely event someone > comes along with a legitimate reason for KVM_EXIT_ON_MISSING to play nice with > KVM_MEM_READONLY. Gotcha, I'll go ahead and make the flags incompatible for the next version. Thanks for the tidbit on how RO memslots are used Oliver- I didn't know that we expect faults on these to be so rare.
