Re: [RFC PATCH 5/6] KVM: x86/mmu: Avoid memslot lookup in rmap_add

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

On 13/08/21 22:35, David Matlack wrote:
> Avoid the memslot lookup in rmap_add by passing it down from the fault
> handling code to mmu_set_spte and then to rmap_add.
>
> No functional change intended.
>
> Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>

I think before doing this we should take another look at the aguments
for make_spte, set_spte and mmu_set_spte.  St

static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
                        u64 *sptep, unsigned int pte_access, bool write_fault,
                        int level, gfn_t gfn, kvm_pfn_t pfn, bool speculative,
                        bool host_writable)

static int set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
                    u64 *sptep, unsigned int pte_access, int level,
                    gfn_t gfn, kvm_pfn_t pfn, bool speculative,
                    bool can_unsync, bool host_writable)

int make_spte(struct kvm_vcpu *vcpu, unsigned int pte_access, int level,
                     gfn_t gfn, kvm_pfn_t pfn, u64 old_spte, bool speculative,
                     bool can_unsync, bool host_writable, bool ad_disabled,
                     u64 *new_spte)

In particular:

- set_spte should be inlined in its two callers.  The SET_SPTE_*
flags are overkill if both functions can just call make_spte+mmu_spte_update:
mmu_set_spte can check *sptep == spte and return RET_PF_SPURIOUS directly,
while SET_SPTE_NEED_REMOTE_TLB_FLUSH can become just a bool that is
returned by make_spte.

- level and ad_disabled can be replaced by a single pointer to struct
kvm_mmu_page (tdp_mmu does not set ad_disabled in page_role_for_level,
but that's not an issue).

- in mmu_set_spte, write_fault, speculative and host_writable are either
false/true/true (prefetching) or fault->write, fault->prefault,
fault->map_writable (pagefault).  So they can be replaced by a single
struct kvm_page_fault pointer, where NULL means false/true/true.  Then
if set_spte is inlined, the ugly bool arguments only remain in make_spte
(minus ad_disabled).

This does not remove the need for a separate slot parameter,
but at least the balance is that there are no extra arguments to
make_spte (two go, level and ad_disabled; two come, sp and slot).

I've started hacking on the above, but didn't quite finish.  I'll
keep patches 4-6 in my queue, but they'll have to wait for 5.15.

Paolo

> ---
>   arch/x86/kvm/mmu/mmu.c         | 29 ++++++++---------------------
>   arch/x86/kvm/mmu/paging_tmpl.h | 12 +++++++++---
>   2 files changed, 17 insertions(+), 24 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index c148d481e9b5..41e2ef8ad09b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1630,16 +1630,15 @@ static bool kvm_test_age_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
>   
>   #define RMAP_RECYCLE_THRESHOLD 1000
>   
> -static void rmap_add(struct kvm_vcpu *vcpu, u64 *spte, gfn_t gfn)
> +static void rmap_add(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
> +		     u64 *spte, gfn_t gfn)
>   {
> -	struct kvm_memory_slot *slot;
>   	struct kvm_mmu_page *sp;
>   	struct kvm_rmap_head *rmap_head;
>   	int rmap_count;
>   
>   	sp = sptep_to_sp(spte);
>   	kvm_mmu_page_set_gfn(sp, spte - sp->spt, gfn);
> -	slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
>   	rmap_head = gfn_to_rmap(gfn, sp->role.level, slot);
>   	rmap_count = pte_list_add(vcpu, spte, rmap_head);
>   
> @@ -2679,9 +2678,9 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>   	return ret;
>   }
>   
> -static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
> -			unsigned int pte_access, bool write_fault, int level,
> -			gfn_t gfn, kvm_pfn_t pfn, bool speculative,
> +static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
> +			u64 *sptep, unsigned int pte_access, bool write_fault,
> +			int level, gfn_t gfn, kvm_pfn_t pfn, bool speculative,
>   			bool host_writable)
>   {
>   	int was_rmapped = 0;
> @@ -2744,24 +2743,12 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>   
>   	if (!was_rmapped) {
>   		kvm_update_page_stats(vcpu->kvm, level, 1);
> -		rmap_add(vcpu, sptep, gfn);
> +		rmap_add(vcpu, slot, sptep, gfn);
>   	}
>   
>   	return ret;
>   }
>   
> -static kvm_pfn_t pte_prefetch_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn,
> -				     bool no_dirty_log)
> -{
> -	struct kvm_memory_slot *slot;
> -
> -	slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn, no_dirty_log);
> -	if (!slot)
> -		return KVM_PFN_ERR_FAULT;
> -
> -	return gfn_to_pfn_memslot_atomic(slot, gfn);
> -}
> -
>   static int direct_pte_prefetch_many(struct kvm_vcpu *vcpu,
>   				    struct kvm_mmu_page *sp,
>   				    u64 *start, u64 *end)
> @@ -2782,7 +2769,7 @@ static int direct_pte_prefetch_many(struct kvm_vcpu *vcpu,
>   		return -1;
>   
>   	for (i = 0; i < ret; i++, gfn++, start++) {
> -		mmu_set_spte(vcpu, start, access, false, sp->role.level, gfn,
> +		mmu_set_spte(vcpu, slot, start, access, false, sp->role.level, gfn,
>   			     page_to_pfn(pages[i]), true, true);
>   		put_page(pages[i]);
>   	}
> @@ -2979,7 +2966,7 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>   			account_huge_nx_page(vcpu->kvm, sp);
>   	}
>   
> -	ret = mmu_set_spte(vcpu, it.sptep, ACC_ALL,
> +	ret = mmu_set_spte(vcpu, fault->slot, it.sptep, ACC_ALL,
>   			   fault->write, fault->goal_level, base_gfn, fault->pfn,
>   			   fault->prefault, fault->map_writable);
>   	if (ret == RET_PF_SPURIOUS)
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 50ade6450ace..653ca44afa58 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -561,6 +561,7 @@ static bool
>   FNAME(prefetch_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>   		     u64 *spte, pt_element_t gpte, bool no_dirty_log)
>   {
> +	struct kvm_memory_slot *slot;
>   	unsigned pte_access;
>   	gfn_t gfn;
>   	kvm_pfn_t pfn;
> @@ -573,8 +574,13 @@ FNAME(prefetch_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>   	gfn = gpte_to_gfn(gpte);
>   	pte_access = sp->role.access & FNAME(gpte_access)(gpte);
>   	FNAME(protect_clean_gpte)(vcpu->arch.mmu, &pte_access, gpte);
> -	pfn = pte_prefetch_gfn_to_pfn(vcpu, gfn,
> +
> +	slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn,
>   			no_dirty_log && (pte_access & ACC_WRITE_MASK));
> +	if (!slot)
> +		return false;
> +
> +	pfn = gfn_to_pfn_memslot_atomic(slot, gfn);
>   	if (is_error_pfn(pfn))
>   		return false;
>   
> @@ -582,7 +588,7 @@ FNAME(prefetch_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>   	 * we call mmu_set_spte() with host_writable = true because
>   	 * pte_prefetch_gfn_to_pfn always gets a writable pfn.
>   	 */
> -	mmu_set_spte(vcpu, spte, pte_access, false, PG_LEVEL_4K, gfn, pfn,
> +	mmu_set_spte(vcpu, slot, spte, pte_access, false, PG_LEVEL_4K, gfn, pfn,
>   		     true, true);
>   
>   	kvm_release_pfn_clean(pfn);
> @@ -749,7 +755,7 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>   		}
>   	}
>   
> -	ret = mmu_set_spte(vcpu, it.sptep, gw->pte_access, fault->write,
> +	ret = mmu_set_spte(vcpu, fault->slot, it.sptep, gw->pte_access, fault->write,
>   			   it.level, base_gfn, fault->pfn, fault->prefault,
>   			   fault->map_writable);
>   	if (ret == RET_PF_SPURIOUS)