Re: [PATCH 5/5] KVM: MMU: fast invalid all mmio sptes

Gleb Natapov
2013-03-17T15:02:47Z

On Fri, Mar 15, 2013 at 11:29:53PM +0800, Xiao Guangrong wrote:
> This patch tries to introduce a very simple and scale way to invalid all
> mmio sptes - it need not walk any shadow pages and hold mmu-lock
>
> KVM maintains a global mmio invalid generation-number which is stored in
> kvm->arch.mmio_invalid_gen and every mmio spte stores the current global
> generation-number into his available bits when it is created
>
> When KVM need zap all mmio sptes, it just simply increase the global
> generation-number. When guests do mmio access, KVM intercepts a MMIO #PF
> then it walks the shadow page table and get the mmio spte. If the
> generation-number on the spte does not equal the global generation-number,
> it will go to the normal #PF handler to update the mmio spte
>
> Since 19 bits are used to store generation-number on mmio spte, the
> generation-number can be round after 33554432 times. It is large enough
> for nearly all most cases, but making the code be more strong, we zap all
> shadow pages when the number is round
>
Very nice idea, but why drop Takuya patches instead of using
kvm_mmu_zap_mmio_sptes() when generation number overflows.

> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> ---
> arch/x86/include/asm/kvm_host.h | 2 +
> arch/x86/kvm/mmu.c | 61 +++++++++++++++++++++++++++++++++------
> arch/x86/kvm/mmutrace.h | 17 +++++++++++
> arch/x86/kvm/paging_tmpl.h | 7 +++-
> arch/x86/kvm/vmx.c | 4 ++
> arch/x86/kvm/x86.c | 6 +--
> 6 files changed, 82 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index ef7f4a5..572398e 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -529,6 +529,7 @@ struct kvm_arch {
> unsigned int n_requested_mmu_pages;
> unsigned int n_max_mmu_pages;
> unsigned int indirect_shadow_pages;
> + unsigned int mmio_invalid_gen;
Why invalid? Should be mmio_valid_gen or mmio_current_get.

> struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];
> /*
> * Hash table of struct kvm_mmu_page.
> @@ -765,6 +766,7 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot);
> void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
> struct kvm_memory_slot *slot,
> gfn_t gfn_offset, unsigned long mask);
> +void kvm_mmu_invalid_mmio_spte(struct kvm *kvm);
Agree with Takuya that kvm_mmu_invalidate_mmio_sptes() is a better name.

> void kvm_mmu_zap_all(struct kvm *kvm);
> unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
> void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int kvm_nr_mmu_pages);
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 13626f4..7093a92 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -234,12 +234,13 @@ static unsigned int get_mmio_spte_generation(u64 spte)
> static void mark_mmio_spte(struct kvm *kvm, u64 *sptep, u64 gfn,
> unsigned access)
> {
> - u64 mask = generation_mmio_spte_mask(0);
> + unsigned int gen = ACCESS_ONCE(kvm->arch.mmio_invalid_gen);
> + u64 mask = generation_mmio_spte_mask(gen);
>
> access &= ACC_WRITE_MASK | ACC_USER_MASK;
> mask |= shadow_mmio_mask | access | gfn << PAGE_SHIFT;
>
> - trace_mark_mmio_spte(sptep, gfn, access, 0);
> + trace_mark_mmio_spte(sptep, gfn, access, gen);
> mmu_spte_set(sptep, mask);
> }
>
> @@ -269,6 +270,34 @@ static bool set_mmio_spte(struct kvm *kvm, u64 *sptep, gfn_t gfn,
> return false;
> }
>
> +static bool check_mmio_spte(struct kvm *kvm, u64 spte)
> +{
> + return get_mmio_spte_generation(spte) ==
> + ACCESS_ONCE(kvm->arch.mmio_invalid_gen);
> +}
> +
> +/*
> + * The caller should protect concurrent access on
> + * kvm->arch.mmio_invalid_gen. Currently, it is used by
> + * kvm_arch_commit_memory_region and protected by kvm->slots_lock.
> + */
> +void kvm_mmu_invalid_mmio_spte(struct kvm *kvm)
> +{
> + /* Ensure update memslot has been completed. */
> + smp_mb();
What barrier this one is paired with?

> +
> + trace_kvm_mmu_invalid_mmio_spte(kvm);
Something wrong with indentation.

> +
> + /*
> + * The very rare case: if the generation-number is round,
> + * zap all shadow pages.
> + */
> + if (unlikely(kvm->arch.mmio_invalid_gen++ == MAX_GEN)) {
> + kvm->arch.mmio_invalid_gen = 0;
> + return kvm_mmu_zap_all(kvm);
> + }
> +}
> +
> static inline u64 rsvd_bits(int s, int e)
> {
> return ((1ULL << (e - s + 1)) - 1) << s;
> @@ -3183,9 +3212,12 @@ static u64 walk_shadow_page_get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr)
> }
>
> /*
> - * If it is a real mmio page fault, return 1 and emulat the instruction
> - * directly, return 0 to let CPU fault again on the address, -1 is
> - * returned if bug is detected.
> + * Return value:
> + * 2: invalid spte is detected then let the real page fault path
> + * update the mmio spte.
> + * 1: it is a real mmio page fault, emulate the instruction directly.
> + * 0: let CPU fault again on the address.
> + * -1: bug is detected.
> */
What about dropping spte and let guest re-fault instead of propagating
new return value up the call chain. If this is slow lets at least define
a enum with descriptive names.

> int handle_mmio_page_fault_common(struct kvm_vcpu *vcpu, u64 addr, bool direct)
> {
> @@ -3200,6 +3232,9 @@ int handle_mmio_page_fault_common(struct kvm_vcpu *vcpu, u64 addr, bool direct)
> gfn_t gfn = get_mmio_spte_gfn(spte);
> unsigned access = get_mmio_spte_access(spte);
>
> + if (unlikely(!check_mmio_spte(vcpu->kvm, spte)))
> + return 2;
> +
> if (direct)
> addr = 0;
>
> @@ -3241,8 +3276,12 @@ static int nonpaging_page_fault(struct kvm_vcpu *vcpu, gva_t gva,
>
> pgprintk("%s: gva %lx error %x\n", __func__, gva, error_code);
>
> - if (unlikely(error_code & PFERR_RSVD_MASK))
> - return handle_mmio_page_fault(vcpu, gva, error_code, true);
> + if (unlikely(error_code & PFERR_RSVD_MASK)) {
> + r = handle_mmio_page_fault(vcpu, gva, error_code, true);
> +
> + if (likely(r != 2))
> + return r;
> + }
>
> r = mmu_topup_memory_caches(vcpu);
> if (r)
> @@ -3318,8 +3357,12 @@ static int tdp_page_fault(struct kvm_vcpu *vcpu, gva_t gpa, u32 error_code,
> ASSERT(vcpu);
> ASSERT(VALID_PAGE(vcpu->arch.mmu.root_hpa));
>
> - if (unlikely(error_code & PFERR_RSVD_MASK))
> - return handle_mmio_page_fault(vcpu, gpa, error_code, true);
> + if (unlikely(error_code & PFERR_RSVD_MASK)) {
> + r = handle_mmio_page_fault(vcpu, gpa, error_code, true);
> +
> + if (likely(r != 2))
> + return r;
> + }
>
> r = mmu_topup_memory_caches(vcpu);
> if (r)
> diff --git a/arch/x86/kvm/mmutrace.h b/arch/x86/kvm/mmutrace.h
> index f5b62a7..811254c 100644
> --- a/arch/x86/kvm/mmutrace.h
> +++ b/arch/x86/kvm/mmutrace.h
> @@ -276,6 +276,23 @@ TRACE_EVENT(
> __spte_satisfied(old_spte), __spte_satisfied(new_spte)
> )
> );
> +
> +TRACE_EVENT(
> + kvm_mmu_invalid_mmio_spte,
> + TP_PROTO(struct kvm *kvm),
> + TP_ARGS(kvm),
> +
> + TP_STRUCT__entry(
> + __field(unsigned int, mmio_invalid_gen)
> + ),
> +
> + TP_fast_assign(
> + __entry->mmio_invalid_gen = kvm->arch.mmio_invalid_gen;
> + ),
> +
> + TP_printk("mmio_invalid_gen %x", __entry->mmio_invalid_gen
> + )
> +);
> #endif /* _TRACE_KVMMMU_H */
>
> #undef TRACE_INCLUDE_PATH
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 2c48e5f..c81f949 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -552,9 +552,12 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
>
> pgprintk("%s: addr %lx err %x\n", __func__, addr, error_code);
>
> - if (unlikely(error_code & PFERR_RSVD_MASK))
> - return handle_mmio_page_fault(vcpu, addr, error_code,
> + if (unlikely(error_code & PFERR_RSVD_MASK)) {
> + r = handle_mmio_page_fault(vcpu, addr, error_code,
> mmu_is_nested(vcpu));
> + if (likely(r != 2))
> + return r;
> + };
>
> r = mmu_topup_memory_caches(vcpu);
> if (r)
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 54fdb76..ca8e9a3 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5159,6 +5159,10 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
> if (likely(ret == 1))
> return x86_emulate_instruction(vcpu, gpa, 0, NULL, 0) ==
> EMULATE_DONE;
> +
> + if (unlikely(ret == 2))
> + return kvm_mmu_page_fault(vcpu, gpa, 0, NULL, 0);
> +
> if (unlikely(!ret))
> return 1;
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 81fb3c3..5b6d2a0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6996,10 +6996,8 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> * If memory slot is created, or moved, we need to clear all
> * mmio sptes.
> */
> - if ((change == KVM_MR_CREATE) || (change == KVM_MR_MOVE)) {
> - kvm_mmu_zap_all(kvm);
> - kvm_reload_remote_mmus(kvm);
> - }
> + if ((change == KVM_MR_CREATE) || (change == KVM_MR_MOVE))
> + kvm_mmu_invalid_mmio_spte(kvm);
> }
>
> void kvm_arch_flush_shadow_all(struct kvm *kvm)
> --
> 1.7.7.6

--
Gleb.