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

From: David Matlack
Date: Tue Sep 28 2021 - 19:20:50 EST

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@xxxxxxxxxx>
> ---
> 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
>