Re: [PATCH v2] KVM: Aggressively drop and reacquire mmu_lock during CLEAR_DIRTY_LOG

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 04, 2024, David Matlack wrote:
> > I don't love the idea of adding more arch specific MMU behavior (going the wrong
> > direction), but it doesn't seem like an unreasonable approach in this case.
> 
> I wonder if this is being overly cautious.

Probably.  "Lazy" is another word for it ;-)

> I would expect only more benefit on architectures that more aggressively take
> the mmu_lock on vCPU threads during faults. The more lock acquisition on vCPU
> threads, the more this patch will help reduce vCPU starvation during
> CLEAR_DIRTY_LOG.
> 
> Hm, perhaps testing with ept=N (which will use the write-lock for even
> dirty logging faults) would be a way to increase confidence in the
> effect on other architectures?

Turning off the TDP MMU would be more representative, just manually disable the
fast-path, e.g.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 992e651540e8..532c24911f39 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3371,7 +3371,7 @@ static bool page_fault_can_be_fast(struct kvm_page_fault *fault)
         * Note, instruction fetches and writes are mutually exclusive, ignore
         * the "exec" flag.
         */
-       return fault->write;
+       return false;//fault->write;
 }
 
 /*