linux-kernel - Re: [PATCH v2 8/9] KVM: x86/mmu: Make kvm_is_mmio

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Date:   Tue, 12 Apr 2022 19:39:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Dunn <daviddunn@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH v2 8/9] KVM: x86/mmu: Make kvm_is_mmio_pfn usable outside
 of spte.c

On Mon, Mar 21, 2022, Ben Gardon wrote:
> Export kvm_is_mmio_pfn from spte.c. It will be used in a subsequent
> commit for in-place lpage promotion when disabling dirty logging.

Rather than force the promotion path to call kvm_is_mmio_pfn(), what about:

  a. Truly exporting the helper, i.e. EXPORT_SYMBOL_GPL
  b. Move this patch earlier in the series, before "KVM: x86/mmu: Factor out part of
     vmx_get_mt_mask which does not depend on vcpu"
  c. In the same patch, drop the "is_mmio" param from kvm_x86_ops.get_mt_mask()
     and have vmx_get_mt_mask() call it directly.

That way the call to kvm_is_mmio_pfn() is avoided when running on AMD hosts
(ignoring the shadow_me_mask thing, which I have a separate tweak for).  The
worst case scenario for a lookup is actually quite expensive, e.g. retpoline and
a spinlock.

> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---
>  arch/x86/kvm/mmu/spte.c | 2 +-
>  arch/x86/kvm/mmu/spte.h | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 45e9c0c3932e..8e9b827c4ed5 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -69,7 +69,7 @@ u64 make_mmio_spte(struct kvm_vcpu *vcpu, u64 gfn, unsigned int access)
>  	return spte;
>  }
>  
> -static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
> +bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
>  {
>  	if (pfn_valid(pfn))
>  		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn)) &&
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index cee02fe63429..e058a85e6c66 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -443,4 +443,5 @@ u64 kvm_mmu_changed_pte_notifier_make_spte(u64 old_spte, kvm_pfn_t new_pfn);
>  
>  void kvm_mmu_reset_all_pte_masks(void);
>  
> +bool kvm_is_mmio_pfn(kvm_pfn_t pfn);
>  #endif
> -- 
> 2.35.1.894.gb6a874cedc-goog
>