On Tue, Jun 04, 2024, Oliver Upton wrote: > On Tue, Jun 04, 2024 at 03:20:20PM -0700, James Houghton wrote: > > On Fri, May 31, 2024 at 12:18 PM Oliver Upton <oliver.upton@xxxxxxxxx> wrote: > > > > > > On Fri, May 31, 2024 at 12:11:33PM -0700, Oliver Upton wrote: > > > > On Wed, May 29, 2024 at 06:05:09PM +0000, James Houghton wrote: > > > Oh, and the WARN_ON() in kvm_pgtable_stage2_test_clear_young() is bogus > > > now. Maybe demote it to: > > > > > > r = kvm_pgtable_walk(...); > > > WARN_ON_ONCE(r && r != -EAGAIN); > > > > Oh, indeed, thank you. Just to make sure -- does it make sense to > > retry the cmpxchg if it fails? For example, the way I have it now for > > x86[1], we retry the cmpxchg if the spte is still a leaf, otherwise we > > move on to the next one having done nothing. Does something like that > > make sense for arm64? > > At least for arm64 I do not see a need for retry. The only possible > races are: > > - A stage-2 fault handler establishing / adjusting the mapping for the > GFN. If the guest is directly accessing the GFN in question, what's > the point of wiping out AF? > > Even when returning -EAGAIN we've already primed stage2_age_data::young, > so we report the correct state back to the primary MMU. > > - Another kvm_age_gfn() trying to age the same GFN. I haven't even > looked to see if this is possible from the primary MMU POV, but in > theory one of the calls will win the race and clear AF. > > Given Yu's concerns about making pending writers wait, we should take > every opportunity to bail on the walk. +1. The x86 path that retries is, for all intents and purposes, limited to Intel CPUs that don't support EPT A/D bits, i.e. to pre-HSW CPUs. I wouldn't make any decisions based on that code.
