linux-kernel - Re: [PATCH v2 3/9] KVM: x86/mmu: Factor shadow_zero_check out of __make

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Date:   Tue, 12 Apr 2022 15:52:03 +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 3/9] KVM: x86/mmu: Factor shadow_zero_check out of
 __make_spte

On Mon, Mar 21, 2022, Ben Gardon wrote:
> In the interest of devloping a version of __make_spte that can function
> without a vCPU pointer, factor out the shadow_zero_mask to be an
> additional argument to the function.
> 
> No functional change intended.
> 
> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---
>  arch/x86/kvm/mmu/spte.c | 10 ++++++----
>  arch/x86/kvm/mmu/spte.h |  2 +-
>  2 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 931cf93c3b7e..ef2d85577abb 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -94,7 +94,7 @@ bool __make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>  		 const struct kvm_memory_slot *slot, unsigned int pte_access,
>  		 gfn_t gfn, kvm_pfn_t pfn, u64 old_spte, bool prefetch,
>  		 bool can_unsync, bool host_writable, u64 mt_mask,
> -		 u64 *new_spte)
> +		 struct rsvd_bits_validate *shadow_zero_check, u64 *new_spte)

Can we name the new param "rsvd_bits"?  As mentioned in the other patch, it's not
a pure "are these bits zero" check.

>  {
>  	int level = sp->role.level;
>  	u64 spte = SPTE_MMU_PRESENT_MASK;
> @@ -177,9 +177,9 @@ bool __make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>  	if (prefetch)
>  		spte = mark_spte_for_access_track(spte);
>  
> -	WARN_ONCE(is_rsvd_spte(&vcpu->arch.mmu->shadow_zero_check, spte, level),
> +	WARN_ONCE(is_rsvd_spte(shadow_zero_check, spte, level),
>  		  "spte = 0x%llx, level = %d, rsvd bits = 0x%llx", spte, level,
> -		  get_rsvd_bits(&vcpu->arch.mmu->shadow_zero_check, spte, level));
> +		  get_rsvd_bits(shadow_zero_check, spte, level));
>  
>  	if ((spte & PT_WRITABLE_MASK) && kvm_slot_dirty_track_enabled(slot)) {
>  		/* Enforced by kvm_mmu_hugepage_adjust. */
> @@ -199,10 +199,12 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>  {
>  	u64 mt_mask = static_call(kvm_x86_get_mt_mask)(vcpu, gfn,
>  						       kvm_is_mmio_pfn(pfn));
> +	struct rsvd_bits_validate *shadow_zero_check =
> +			&vcpu->arch.mmu->shadow_zero_check;
>  
>  	return __make_spte(vcpu, sp, slot, pte_access, gfn, pfn, old_spte,
>  			   prefetch, can_unsync, host_writable, mt_mask,
> -			   new_spte);
> +			   shadow_zero_check, new_spte);

I don't see any reason to snapshot the reserved bits, IMO this is much more
readable overall:

	u64 mt_mask = static_call(kvm_x86_get_mt_mask)(vcpu, gfn,
						       kvm_is_mmio_pfn(pfn));

	return __make_spte(vcpu->kvm, sp, slot, pte_access, gfn, pfn, old_spte,
			   prefetch, can_unsync, host_writable, mt_mask,
			   &vcpu->arch.mmu->shadow_zero_check, new_spte);

And it avoids propagating the shadow_zero_check naming.

> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index d051f955699e..e8a051188eb6 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -414,7 +414,7 @@ bool __make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>  		 const struct kvm_memory_slot *slot, unsigned int pte_access,
>  		 gfn_t gfn, kvm_pfn_t pfn, u64 old_spte, bool prefetch,
>  		 bool can_unsync, bool host_writable, u64 mt_mask,
> -		 u64 *new_spte);
> +		 struct rsvd_bits_validate *shadow_zero_check, u64 *new_spte);
>  bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>  	       const struct kvm_memory_slot *slot,
>  	       unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,
> -- 
> 2.35.1.894.gb6a874cedc-goog
>