Re: [PATCH v3 31/31] KVM: MMU: make spte an in-out argument in make_spte

[David Matlack <dmatlack@google.com>]

On Fri, Sep 24, 2021 at 12:31:52PM -0400, Paolo Bonzini wrote:
> Pass the old SPTE in the same variable that receives the new SPTE.  This
> reduces the number of arguments from 11 to 10.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/mmu/mmu.c         | 18 ++++++++----------
>  arch/x86/kvm/mmu/paging_tmpl.h |  2 +-
>  arch/x86/kvm/mmu/spte.c        |  7 ++++---
>  arch/x86/kvm/mmu/spte.h        |  2 +-
>  arch/x86/kvm/mmu/tdp_mmu.c     |  9 +++++----
>  5 files changed, 19 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 91292009780a..b363433bcd2c 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2682,8 +2682,8 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
>  	int was_rmapped = 0;
>  	int ret = RET_PF_FIXED;
>  	bool flush = false;
> +	u64 spte = *sptep;
>  	bool wrprot;
> -	u64 spte;
>  
>  	/* Prefetching always gets a writable pfn.  */
>  	bool host_writable = !fault || fault->map_writable;
> @@ -2691,35 +2691,33 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
>  	bool write_fault = fault && fault->write;
>  
>  	pgprintk("%s: spte %llx write_fault %d gfn %llx\n", __func__,
> -		 *sptep, write_fault, gfn);
> +		 spte, write_fault, gfn);
>  
>  	if (unlikely(is_noslot_pfn(pfn))) {
>  		mark_mmio_spte(vcpu, sptep, gfn, pte_access);
>  		return RET_PF_EMULATE;
>  	}
>  
> -	if (is_shadow_present_pte(*sptep)) {
> +	if (is_shadow_present_pte(spte)) {
>  		/*
>  		 * If we overwrite a PTE page pointer with a 2MB PMD, unlink
>  		 * the parent of the now unreachable PTE.
>  		 */
> -		if (level > PG_LEVEL_4K && !is_large_pte(*sptep)) {
> +		if (level > PG_LEVEL_4K && !is_large_pte(spte)) {
>  			struct kvm_mmu_page *child;
> -			u64 pte = *sptep;
> -
> -			child = to_shadow_page(pte & PT64_BASE_ADDR_MASK);
> +			child = to_shadow_page(spte & PT64_BASE_ADDR_MASK);
>  			drop_parent_pte(child, sptep);
>  			flush = true;
> -		} else if (pfn != spte_to_pfn(*sptep)) {
> +		} else if (pfn != spte_to_pfn(spte)) {
>  			pgprintk("hfn old %llx new %llx\n",
> -				 spte_to_pfn(*sptep), pfn);
> +				 spte_to_pfn(spte), pfn);
>  			drop_spte(vcpu->kvm, sptep);
>  			flush = true;
>  		} else
>  			was_rmapped = 1;
>  	}
>  
> -	wrprot = make_spte(vcpu, sp, slot, pte_access, gfn, pfn, *sptep, speculative,
> +	wrprot = make_spte(vcpu, sp, slot, pte_access, gfn, pfn, speculative,
>  			   true, host_writable, &spte);
>  
>  	if (*sptep == spte) {
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index d8889e02c4b7..88551cfd06c6 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -1130,7 +1130,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>  		host_writable = spte & shadow_host_writable_mask;
>  		slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
>  		make_spte(vcpu, sp, slot, pte_access, gfn,
> -			  spte_to_pfn(spte), spte, true, false,
> +			  spte_to_pfn(spte), true, false,
>  			  host_writable, &spte);
>  
>  		flush |= mmu_spte_update(sptep, spte);
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 871f6114b0fa..91525388032e 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -92,10 +92,11 @@ static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
>  bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>  	       struct kvm_memory_slot *slot,
>  	       unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,
> -	       u64 old_spte, bool speculative, bool can_unsync,
> -	       bool host_writable, u64 *new_spte)
> +	       bool speculative, bool can_unsync,
> +	       bool host_writable, u64 *sptep)

I'd prefer a different name since `sptep` has specific meaning
throughout the mmu code. (It's the address of the spte in the page
table.)

Case in point, I was going to suggest we can get rid of struct
kvm_mmu_page since it can be derived from the sptep and then realized
how wrong that was :).

Instead of receiving the new spte as a parameter what do you think about
changing make_spte to return the new spte? I think that would make the
code more readable (but won't reduce the number of arguments because
you'd have to add wrprot).

>  {
>  	int level = sp->role.level;
> +	u64 old_spte = *sptep;
>  	u64 spte = SPTE_MMU_PRESENT_MASK;
>  	bool wrprot = false;
>  
> @@ -187,7 +188,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>  		mark_page_dirty_in_slot(vcpu->kvm, slot, gfn);
>  	}
>  
> -	*new_spte = spte;
> +	*sptep = spte;
>  	return wrprot;
>  }
>  
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 7c0b09461349..231531c6015a 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -337,7 +337,7 @@ static inline u64 get_mmio_spte_generation(u64 spte)
>  bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>  	       struct kvm_memory_slot *slot,
>  	       unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,
> -	       u64 old_spte, bool speculative, bool can_unsync,
> +	       bool speculative, bool can_unsync,
>  	       bool host_writable, u64 *new_spte);
>  u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled);
>  u64 make_mmio_spte(struct kvm_vcpu *vcpu, u64 gfn, unsigned int access);
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 953f24ded6bc..29b739c7bba4 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -903,13 +903,14 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
>  	bool wrprot = false;
>  
>  	WARN_ON(sp->role.level != fault->goal_level);
> -	if (unlikely(!fault->slot))
> +	if (unlikely(!fault->slot)) {
>  		new_spte = make_mmio_spte(vcpu, iter->gfn, ACC_ALL);
> -	else
> +	} else {
> +		new_spte = iter->old_spte;
>  		wrprot = make_spte(vcpu, sp, fault->slot, ACC_ALL, iter->gfn,
> -					 fault->pfn, iter->old_spte, fault->prefault, true,
> +					 fault->pfn, fault->prefault, true,
>  					 fault->map_writable, &new_spte);
> -
> +	}
>  	if (new_spte == iter->old_spte)
>  		ret = RET_PF_SPURIOUS;
>  	else if (!tdp_mmu_set_spte_atomic(vcpu->kvm, iter, new_spte))
> -- 
> 2.27.0
>