Am 23/09/2022 um 22:28 schrieb David Matlack: > On Fri, Sep 23, 2022 at 12:25:00PM -0700, Jim Mattson wrote: >> On Fri, Sep 23, 2022 at 3:16 AM Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote: >>> >>> Because of this, when the guest clears the accessed bit in its nested EPT entries, KVM doesn't >>> notice/intercept it and corresponding EPT sptes remain the same, thus later the guest access to >>> the memory is not intercepted and because of this doesn't turn back >>> the accessed bit in the guest EPT tables. >> >> Does the guest execute an INVEPT after clearing the accessed bit? > > No, that's the problem. In L1, access_tracking_perf_test is using > page_idle to mark guest memory as idle, which results in clear_young() > notifiers being sent to KVM clear access bits. clear_young() is > explicitly allowed to omit flushes, so KVM happily obliges. > > /* > * clear_young is a lightweight version of clear_flush_young. Like the > * latter, it is supposed to test-and-clear the young/accessed bitflag > * in the secondary pte, but it may omit flushing the secondary tlb. > */ > int (*clear_young)(struct mmu_notifier *subscription, > struct mm_struct *mm, > unsigned long start, > unsigned long end); > > We could modify page_idle so that KVM performs TLB flushes. For example, > add a mechanism for userspace to trigger a TLB flush. Or change > page_idle to use clear_flush_young() (although that would be incredibly > expensive since page_idle only allows clearing one pfn at a time). But > I'm not sure creating a new userspace API just for this test is really > worth it, especially with multigen LRU coming soon. > Thank you David and Jim for the feedback. I sent a patch converting the assertion in warning here: https://lkml.org/lkml/2022/9/26/238 Thank you, Emanuele
