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    SubjectRe: [PATCH 2/3] KVM: arm64: Don't map PUD huge page if it's not available
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    Hi Marc,

    On 10/25/20 9:05 PM, Marc Zyngier wrote:
    > On Sun, 25 Oct 2020 01:27:38 +0100,
    > Gavin Shan <gshan@redhat.com> wrote:
    >>
    >> PUD huge page isn't available when CONFIG_ARM64_4K_PAGES is disabled.
    >> In this case, we needn't try to map the memory through PUD huge pages
    >> to save some CPU cycles in the hot path.
    >>
    >> This also corrects the code style issue, which was introduced by
    >> commit <523b3999e5f6> ("KVM: arm64: Try PMD block mappings if PUD mappings
    >> are not supported").
    >>
    >> Signed-off-by: Gavin Shan <gshan@redhat.com>
    >> ---
    >> arch/arm64/kvm/mmu.c | 4 +++-
    >> 1 file changed, 3 insertions(+), 1 deletion(-)
    >>
    >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
    >> index a816cb8e619b..0f51585adc04 100644
    >> --- a/arch/arm64/kvm/mmu.c
    >> +++ b/arch/arm64/kvm/mmu.c
    >> @@ -787,9 +787,11 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
    >> vma_shift = PAGE_SHIFT;
    >> }
    >>
    >> +#ifdef CONFIG_ARM64_4K_PAGES
    >> if (vma_shift == PUD_SHIFT &&
    >> !fault_supports_stage2_huge_mapping(memslot, hva, PUD_SIZE))
    >> - vma_shift = PMD_SHIFT;
    >> + vma_shift = PMD_SHIFT;
    >> +#endif
    >>
    >> if (vma_shift == PMD_SHIFT &&
    >> !fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) {
    >
    >
    > I really don't buy the "CPU cycles" argument here either. Can you
    > actually measure any difference here?
    >
    > You have taken a fault, gone through a full guest exit, triaged it,
    > and are about to into a page mapping operation which may result in a
    > TLBI, and reenter the guest. It only happen a handful of times per
    > page over the lifetime of the guest unless you start swapping. Hot
    > path? I don't think so.
    >

    Thanks for the explanation. Agreed and I will drop this in v2.

    Thanks,
    Gavin

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