Re: [PATCH 2/3] KVM: x86/mmu: Drop a redundant remote TLB flush in kvm_zap_gfn_range()
From: Maciej S. Szmigiero
Date: Tue Oct 26 2021 - 10:23:45 EST
On 25.10.2021 17:39, Sean Christopherson wrote:
On Fri, Oct 22, 2021, Maciej S. Szmigiero wrote:
On 22.10.2021 03:00, Sean Christopherson wrote:
Remove an unnecessary remote TLB flush in kvm_zap_gfn_range() now that
said function holds mmu_lock for write for its entire duration. The
flush was added by the now-reverted commit to allow TDP MMU to flush while
holding mmu_lock for read, as the transition from write=>read required
dropping the lock and thus a pending flush needed to be serviced.
Fixes: 5a324c24b638 ("Revert "KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock"")
Cc: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Cc: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
Cc: Ben Gardon <bgardon@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
arch/x86/kvm/mmu/mmu.c | 3 ---
1 file changed, 3 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index f82b192bba0b..e8b8a665e2e9 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5700,9 +5700,6 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
end - 1, true, flush);
}
}
- if (flush)
- kvm_flush_remote_tlbs_with_address(kvm, gfn_start,
- gfn_end - gfn_start);
}
if (is_tdp_mmu_enabled(kvm)) {
Unfortunately, it seems that a pending flush from __kvm_zap_rmaps()
can be reset back to false by the following line:
flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, gfn_start, gfn_end, flush);
kvm_tdp_mmu_zap_gfn_range() calls __kvm_tdp_mmu_zap_gfn_range with
"can_yield" set to true, which passes it to zap_gfn_range, which has
this code:
if (can_yield &&
tdp_mmu_iter_cond_resched(kvm, &iter, flush, shared)) {
flush = false;
continue;
}
That's working by design. If the MMU (legacy or TDP) yields during zap, it _must_
flush before dropping mmu_lock so that any SPTE modifications are guaranteed to be
observed by all vCPUs. Clearing "flush" is deliberate/correct as another is flush
is needed if and only if additional SPTE modifications are made.
Got it, thanks.
Maciej