Messages in this thread | | | Date | Thu, 31 Aug 2023 11:50:20 -0500 | Subject | Re: [PATCH 4/8] KVM: gmem: protect kvm_mmu_invalidate_end() | From | "Kalra, Ashish" <> |
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Hello Sean,
On 8/22/2023 6:17 PM, Sean Christopherson wrote: > On Mon, Aug 21, 2023, Ashish Kalra wrote: >> Hello Mingwei & Sean, >> >> On 8/18/2023 9:08 PM, Mingwei Zhang wrote: >> The maximum hits are seen with shmem_fallocate and madvise, which we believe >> are response to shared<->private >> GHCB page-state-chage requests. discard=both handles discard both for >> private and shared memory, so freeing shared memory >> via fallocate(shared_memfd, FALLOC_FL_PUNCH_HOLE, ...) would trigger the >> notifiers when freeing shared pages after guest converts a GPA to >> private. >> >> Now, as with SNP+guest_memfd, guest private memory is not mapped in host >> anymore, so i added a generic fix (instead of Sean's proposed patch of >> checking for SNP guest inside sev_guest_memory_reclaimed()): >> >> --- a/virt/kvm/kvm_main.c >> +++ b/virt/kvm/kvm_main.c >> @@ -593,6 +593,9 @@ static __always_inline int __kvm_handle_hva_range(struct >> kvm *kvm, >> unsigned long hva_start, hva_end; >> >> slot = container_of(node, struct kvm_memory_slot, >> hva_node[slots->node_idx]); >> + if (kvm_slot_can_be_private(slot)) { >> + continue; >> + } >> hva_start = max(range->start, slot->userspace_addr); >> hva_end = min(range->end, slot->userspace_addr + >> (slot->npages << >> PAGE_SHIFT)); > > ... > >> As expected, the SEV hook is not invoked for the guest private memory pages >> (no more invalidation from shmem_fallocate() + madvise()). >> >> Isn't it better to skip invoking the KVM MMU invalidation notifier when the >> invalidated range belongs to guest private memory ? > > Oooh, you're running into problems where KVM blasts both the private and shared > mappings even though invalidations from the mmu_notifier are shared-only by > definition. > > The answer is "yes", but simply skipping slots that _can_ be private is wrong, > as KVM still needs to zap any shared mappings. I have a plan[*], but I completely > spaced on incorporating the idea into the gmem RFC. I'll add that to the "list > of todos for merging gmem", which I need to get sent out asap. > > https://lore.kernel.org/all/ZJX0hk+KpQP0KUyB@google.com
Looking at your gmem TODO's post, i don't see anything specific for this support:
https://lore.kernel.org/kvm/ZOjpIL0SFH+E3Dj4@google.com/
Thanks, Ashish
> >>> In fact, AFAIC, SNP VM does not track whether each page is previously >>> shared, isn't it? If a page was previously shared and was written by the >>> host kernel or devices before it was changed to private. No one tracks it >>> and dirty caches are there! >> >> The skipped invalidation here covered the case Mingwei mentioned above, >> where the pages are changed from private->shared and subsequent freeing of >> shared pages triggered the invalidation. >> >> But, then why are we concerned about this, i thought we have concerns about >> the case where the dirty cache lines contain encrypted guest data ? > > Yes, that's my understanding as well (assuming by "this" you mean the case where > the CPU cache has dirty lines for _shared_ addresses). >
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