Re: [PATCH 2/3] KVM: x86/mmu: Drop a redundant remote TLB flush in kvm_zap_gfn_range()

["Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>]

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@redhat.com>
> Cc: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> Cc: Ben Gardon <bgardon@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   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;
> }

Thanks,
Maciej