Re: [PATCH 2/2] KVM: x86/mmu: Retry fault before acquiring mmu_lock if mapping is changing

[Huang, Kai]

On Thu, 2023-08-24 at 19:07 -0700, Sean Christopherson wrote:
> Retry page faults without acquiring mmu_lock if the resolved hva is covered
> by an active invalidation.  Contending for mmu_lock is especially
> problematic on preemptible kernels as the mmu_notifier invalidation task
> will yield mmu_lock (see rwlock_needbreak()), delay the in-progress
> invalidation, and ultimately increase the latency of resolving the page
> fault.  And in the worst case scenario, yielding will be accompanied by a
> remote TLB flush, e.g. if the invalidation covers a large range of memory
> and vCPUs are accessing addresses that were already zapped.
> 
> Alternatively, the yielding issue could be mitigated by teaching KVM's MMU
> iterators to perform more work before yielding, but that wouldn't solve
> the lock contention and would negatively affect scenarios where a vCPU is
> trying to fault in an address that is NOT covered by the in-progress
> invalidation.
> 
> Reported-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/ZNnPF4W26ZbAyGto@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Acked-by: Kai Huang <kai.huang@xxxxxxxxx>

Nit below ...

> ---
>  arch/x86/kvm/mmu/mmu.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 1a5a1e7d1eb7..8e2e07ed1a1b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4334,6 +4334,9 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>  	if (unlikely(!fault->slot))
>  		return kvm_handle_noslot_fault(vcpu, fault, access);
>  
> +	if (mmu_invalidate_retry_hva(vcpu->kvm, fault->mmu_seq, fault->hva))
> +		return RET_PF_RETRY;
> +

... Perhaps a comment saying this is to avoid unnecessary MMU lock contention
would be nice.  Otherwise we have is_page_fault_stale() called later within the
MMU lock.  I suppose people only tend to use git blamer when they cannot find
answer in the code :-)
 
>  	return RET_PF_CONTINUE;
>  }
>  

Btw, currently fault->mmu_seq is set in kvm_faultin_pfn(), which happens after
fast_page_fault().  Conceptually, should we move this to even before
fast_page_fault() because I assume the range zapping should also apply to the
cases that fast_page_fault() handles?