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From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Sergey Senozhatsky <senozhatsky@google.com>,
	Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH] KVM: x86/mmu: Complete prefetch for trailing SPTEs for direct, legacy MMU
Date: Thu, 23 Sep 2021 18:27:03 +0200	[thread overview]
Message-ID: <c47ae22f-39ba-7f6b-c9c4-105a2e4c026d@redhat.com> (raw)
In-Reply-To: <20210818235615.2047588-1-seanjc@google.com>

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@google.com>
> Cc: Ben Gardon <bgardon@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> 
> 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


      parent reply	other threads:[~2021-09-23 16:27 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-18 23:56 Sean Christopherson
2021-08-19  4:15 ` Sergey Senozhatsky
2021-08-25 22:49 ` Lai Jiangshan
2021-08-26 21:35   ` Ben Gardon
2021-09-23 16:27 ` Paolo Bonzini [this message]

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