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

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Date:   Mon, 15 Mar 2021 18:46:06 +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 17:56, Gavin Shan wrote:
> Hi Keqian,
> 
> On 3/15/21 7:25 PM, Keqian Zhu wrote:
>> 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.
>>
> 
> Yeah, it's correct. Besides, the @logging_active is determined
> based on the passed memory slot. It means user_mem_abort() can't
> support memory range which spans multiple memory slot.
> 
>>> 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.
>>
> 
> I guess you need some comments here? If so, I would add something
> like below in v2:
> 
>     /*
>      * gfn_to_pfn_prot() can be used either with unnecessary overhead
>      * introduced to locate the memory slot because the memory slot is
>      * always fixed even @gfn is adjusted for huge pages.
>      */
Looks good.

See comments above "smp_rmb();", and actually my meaning is that we should change "gfn_to_pfn_prot"
to "__gfn_to_pfn_memslot" :)

Thanks,
Keqian

> 
>>>       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,
> Gavin
> 
> 
> .
>