linux-kernel - Re: [PATCH 4/4] KVM: arm64: Don't retrieve memory slot again in page fault handler

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Message-ID: <30073114-339f-33dd-0168-b4d6bfbe88bc@huawei.com>
Date:   Mon, 15 Mar 2021 16:25:19 +0800
From:   Keqian Zhu <zhukeqian1@...wei.com>
To:     Gavin Shan <gshan@...hat.com>, <kvmarm@...ts.cs.columbia.edu>
CC:     <maz@...nel.org>, <will@...nel.org>,
        <linux-kernel@...r.kernel.org>, <shan.gavin@...il.com>
Subject: Re: [PATCH 4/4] KVM: arm64: Don't retrieve memory slot again in page
 fault handler

Hi Gavin,

On 2021/3/15 12:18, Gavin Shan wrote:
> We needn't retrieve the memory slot again in user_mem_abort() because
> the corresponding memory slot has been passed from the caller. This
I think you are right, though fault_ipa will be adjusted when we try to use block mapping,
the fault_supports_stage2_huge_mapping() makes sure we're not trying to map anything
not covered by the memslot, so the adjusted fault_ipa still belongs to the memslot.

> would save some CPU cycles. For example, the time used to write 1GB
> memory, which is backed by 2MB hugetlb pages and write-protected, is
> dropped by 6.8% from 928ms to 864ms.
> 
> Signed-off-by: Gavin Shan <gshan@...hat.com>
> ---
>  arch/arm64/kvm/mmu.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index a5a8ade9fde4..4a4abcccfafb 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -846,7 +846,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	 */
>  	smp_rmb();
>  
> -	pfn = gfn_to_pfn_prot(kvm, gfn, write_fault, &writable);
> +	pfn = __gfn_to_pfn_memslot(memslot, gfn, false, NULL,
> +				   write_fault, &writable, NULL);
It's better to update the code comments at same time.

>  	if (pfn == KVM_PFN_ERR_HWPOISON) {
>  		kvm_send_hwpoison_signal(hva, vma_shift);
>  		return 0;
> @@ -912,7 +913,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	/* Mark the page dirty only if the fault is handled successfully */
>  	if (writable && !ret) {
>  		kvm_set_pfn_dirty(pfn);
> -		mark_page_dirty(kvm, gfn);
> +		mark_page_dirty_in_slot(kvm, memslot, gfn);
>  	}
>  
>  out_unlock:
> 

Thanks,
Keqian.