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 20:56:17 +1100
From:   Gavin Shan <gshan@...hat.com>
To:     Keqian Zhu <zhukeqian1@...wei.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 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.
	 */

>>   	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