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

[Gavin Shan]

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@xxxxxxxxxx>
> > ---
> >   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