Re: [PATCH] KVM: x86/mmu: Complete prefetch for trailing SPTEs for direct, legacy MMU

[Paolo Bonzini]

On 19/08/21 01:56, Sean Christopherson wrote:
> Make a final call to direct_pte_prefetch_many() if there are "trailing"
> SPTEs to prefetch, i.e. SPTEs for GFNs following the faulting GFN.  The
> call to direct_pte_prefetch_many() in the loop only handles the case
> where there are !PRESENT SPTEs preceding a PRESENT SPTE.
>
> E.g. if the faulting GFN is a multiple of 8 (the prefetch size) and all
> SPTEs for the following GFNs are !PRESENT, the loop will terminate with
> "start = sptep+1" and not prefetch any SPTEs.
>
> Prefetching trailing SPTEs as intended can drastically reduce the number
> of guest page faults, e.g. accessing the first byte of every 4kb page in
> a 6gb chunk of virtual memory, in a VM with 8gb of preallocated memory,
> the number of pf_fixed events observed in L0 drops from ~1.75M to <0.27M.
>
> Note, this only affects memory that is backed by 4kb pages as KVM doesn't
> prefetch when installing hugepages.  Shadow paging prefetching is not
> affected as it does not batch the prefetches due to the need to process
> the corresponding guest PTE.  The TDP MMU is not affected because it
> doesn't have prefetching, yet...
>
> Fixes: 957ed9effd80 ("KVM: MMU: prefetch ptes when intercepted guest #PF")
> Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxx>
> Cc: Ben Gardon <bgardon@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>
> Cc'd Ben as this highlights a potential gap with the TDP MMU, which lacks
> prefetching of any sort.  For large VMs, which are likely backed by
> hugepages anyways, this is a non-issue as the benefits of holding mmu_lock
> for read likely masks the cost of taking more VM-Exits.  But VMs with a
> small number of vCPUs won't benefit as much from parallel page faults,
> e.g. there's no benefit at all if there's a single vCPU.
>
>   arch/x86/kvm/mmu/mmu.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index a272ccbddfa1..daf7df35f788 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2818,11 +2818,13 @@ static void __direct_pte_prefetch(struct kvm_vcpu *vcpu,
>   			if (!start)
>   				continue;
>   			if (direct_pte_prefetch_many(vcpu, sp, start, spte) < 0)
> -				break;
> +				return;
>   			start = NULL;
>   		} else if (!start)
>   			start = spte;
>   	}
> +	if (start)
> +		direct_pte_prefetch_many(vcpu, sp, start, spte);
>   }
>   
>   static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep)

Queued, thanks.

Paolo