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/[email protected]
Looking at your gmem TODO's post, i don't see anything specific for this
support:
https://lore.kernel.org/kvm/[email protected]/
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).
>