linux-kernel - Re: [PATCH 2/3] KVM: arm64: Don't map PUD huge page if it's not available

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Date:   Sun, 25 Oct 2020 10:05:41 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Gavin Shan <gshan@...hat.com>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        will@...nel.org, alexandru.elisei@....com
Subject: Re: [PATCH 2/3] KVM: arm64: Don't map PUD huge page if it's not available

On Sun, 25 Oct 2020 01:27:38 +0100,
Gavin Shan <gshan@...hat.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@...hat.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.

	M.

-- 
Without deviation from the norm, progress is not possible.