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SubjectRe: [PATCH v6 25/29] KVM: Call kvm_arch_flush_shadow_memslot() on the old slot in kvm_invalidate_memslot()
On Tue, Nov 30, 2021, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
>
> kvm_invalidate_memslot() calls kvm_arch_flush_shadow_memslot() on the
> active, but KVM_MEMSLOT_INVALID slot.
> Do it on the inactive (but valid) old slot instead since arch code really
> should not get passed such invalid slot.

One other thing that's worth noting in the changelog is that "old->arch" may have
stale data. IMO that's perfectly ok, but it's definitely a quirk. Ideally KVM
would disallow touching "arch" for an INVALID slot, but that would require another
arch hook if kvm_prepare_memory_region() failed to refresh old->arch if necessary
before restoring it. :-/

Paolo, thoughts on this goofy case? I don't love it, but I dislike having

kvm_arch_flush_shadow_memslot(kvm, invalid_slot);

in the final code even more.

Reviewed-by: Sean Christopherson <seanjc@google.com>

> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> ---
> virt/kvm/kvm_main.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index c57748ee41e8..086f18969bc3 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1632,7 +1632,7 @@ static void kvm_invalidate_memslot(struct kvm *kvm,
> * - gfn_to_hva (kvm_read_guest, gfn_to_pfn)
> * - kvm_is_visible_gfn (mmu_check_root)
> */
> - kvm_arch_flush_shadow_memslot(kvm, working_slot);
> + kvm_arch_flush_shadow_memslot(kvm, old);
>
> /* Was released by kvm_swap_active_memslots, reacquire. */
> mutex_lock(&kvm->slots_arch_lock);

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