Re: [PATCH 2/3] arm/arm64: KVM: Implement Stage-2 page aging

Christoffer Dall
2015-03-12T11:40:17Z

On Wed, Jan 21, 2015 at 06:42:12PM +0000, Marc Zyngier wrote:
> Until now, KVM/arm didn't care much for page aging (who was swapping
> anyway?), and simply provided empty hooks to the core KVM code. With
> server-type systems now being available, things are quite different.
>
> This patch implements very simple support for page aging, by clearing
> the Access flag in the Stage-2 page tables. On access fault, the current
> fault handling will write the PTE or PMD again, putting the Access flag
> back on.
>
> It should be possible to implement a much faster handling for Access
> faults, but that's left for a later patch.
>
> With this in place, performance in VMs is degraded much more gracefully.
>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
> arch/arm/include/asm/kvm_host.h | 13 ++-------
> arch/arm/kvm/mmu.c | 59 ++++++++++++++++++++++++++++++++++++++-
> arch/arm/kvm/trace.h | 33 ++++++++++++++++++++++
> arch/arm64/include/asm/kvm_arm.h | 1 +
> arch/arm64/include/asm/kvm_host.h | 13 ++-------
> 5 files changed, 96 insertions(+), 23 deletions(-)
>
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index 04b4ea0..d6b5b85 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -163,19 +163,10 @@ void kvm_set_spte_hva(struct kvm *kvm, unsigned long hva, pte_t pte);
>
> unsigned long kvm_arm_num_regs(struct kvm_vcpu *vcpu);
> int kvm_arm_copy_reg_indices(struct kvm_vcpu *vcpu, u64 __user *indices);
> +int kvm_age_hva(struct kvm *kvm, unsigned long start, unsigned long end);
> +int kvm_test_age_hva(struct kvm *kvm, unsigned long hva);
>
> /* We do not have shadow page tables, hence the empty hooks */
> -static inline int kvm_age_hva(struct kvm *kvm, unsigned long start,
> - unsigned long end)
> -{
> - return 0;
> -}
> -
> -static inline int kvm_test_age_hva(struct kvm *kvm, unsigned long hva)
> -{
> - return 0;
> -}
> -
> static inline void kvm_arch_mmu_notifier_invalidate_page(struct kvm *kvm,
> unsigned long address)
> {
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index e163a45..ffe89a0 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -1068,6 +1068,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>
> out_unlock:
> spin_unlock(&kvm->mmu_lock);
> + kvm_set_pfn_accessed(pfn);
> kvm_release_pfn_clean(pfn);
> return ret;
> }
> @@ -1102,7 +1103,8 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run)
>
> /* Check the stage-2 fault is trans. fault or write fault */
> fault_status = kvm_vcpu_trap_get_fault_type(vcpu);
> - if (fault_status != FSC_FAULT && fault_status != FSC_PERM) {
> + if (fault_status != FSC_FAULT && fault_status != FSC_PERM &&
> + fault_status != FSC_ACCESS) {
> kvm_err("Unsupported FSC: EC=%#x xFSC=%#lx ESR_EL2=%#lx\n",
> kvm_vcpu_trap_get_class(vcpu),
> (unsigned long)kvm_vcpu_trap_get_fault(vcpu),
> @@ -1237,6 +1239,61 @@ void kvm_set_spte_hva(struct kvm *kvm, unsigned long hva, pte_t pte)
> handle_hva_to_gpa(kvm, hva, end, &kvm_set_spte_handler, &stage2_pte);
> }
>
> +static int kvm_age_hva_handler(struct kvm *kvm, gpa_t gpa, void *data)
> +{
> + pmd_t *pmd;
> + pte_t *pte;
> +
> + pmd = stage2_get_pmd(kvm, NULL, gpa);
> + if (!pmd || pmd_none(*pmd)) /* Nothing there */
> + return 0;
> +
> + if (kvm_pmd_huge(*pmd)) { /* THP, HugeTLB */
> + *pmd = pmd_mkold(*pmd);
> + goto tlbi;

so in this case we'll loop over a huge pmd on a page-by-page basis,
invalidating the tlb each time, right?

Would it be worth checking of the access flag is already clear
(!pmd_young()) and in that case exit without doing tlb invalidation?

In fact, shouldn't we only return 1 if the pmd is indeed young and then
the tlb invalidation will be done once by kvm_flush_remote_tlbs() in
kvm_mmu_notifier_clear_flush_young() ?

I got a little lost looking at how the core mm code uses the return
value, but if I read the x86 and powerpc code correctly, they use it the
way I suggest. Did I get this all wrong?

> + }
> +
> + pte = pte_offset_kernel(pmd, gpa);
> + if (pte_none(*pte))
> + return 0;
> +
> + *pte = pte_mkold(*pte); /* Just a page... */

same with checking if it's young or not... ?

> +tlbi:
> + kvm_tlb_flush_vmid_ipa(kvm, gpa);
> + return 1;
> +}
> +
> +static int kvm_test_age_hva_handler(struct kvm *kvm, gpa_t gpa, void *data)
> +{
> + pmd_t *pmd;
> + pte_t *pte;
> +
> + pmd = stage2_get_pmd(kvm, NULL, gpa);
> + if (!pmd || pmd_none(*pmd)) /* Nothing there */
> + return 0;
> +
> + if (kvm_pmd_huge(*pmd)) /* THP, HugeTLB */
> + return pmd_young(*pmd);
> +
> + pte = pte_offset_kernel(pmd, gpa);
> + if (!pte_none(*pte)) /* Just a page... */
> + return pte_young(*pte);
> +
> + return 0;
> +}
> +
> +int kvm_age_hva(struct kvm *kvm, unsigned long start, unsigned long end)
> +{
> + trace_kvm_age_hva(start, end);
> + return handle_hva_to_gpa(kvm, start, end, kvm_age_hva_handler, NULL);
> +}
> +
> +int kvm_test_age_hva(struct kvm *kvm, unsigned long hva)
> +{
> + trace_kvm_test_age_hva(hva);
> + return handle_hva_to_gpa(kvm, hva, hva, kvm_test_age_hva_handler, NULL);
> +}
> +
> void kvm_mmu_free_memory_caches(struct kvm_vcpu *vcpu)
> {
> mmu_free_memory_cache(&vcpu->arch.mmu_page_cache);
> diff --git a/arch/arm/kvm/trace.h b/arch/arm/kvm/trace.h
> index b6a6e71..364b5382 100644
> --- a/arch/arm/kvm/trace.h
> +++ b/arch/arm/kvm/trace.h
> @@ -203,6 +203,39 @@ TRACE_EVENT(kvm_set_spte_hva,
> TP_printk("mmu notifier set pte hva: %#08lx", __entry->hva)
> );
>
> +TRACE_EVENT(kvm_age_hva,
> + TP_PROTO(unsigned long start, unsigned long end),
> + TP_ARGS(start, end),
> +
> + TP_STRUCT__entry(
> + __field( unsigned long, start )
> + __field( unsigned long, end )
> + ),
> +
> + TP_fast_assign(
> + __entry->start = start;
> + __entry->end = end;
> + ),
> +
> + TP_printk("mmu notifier age hva: %#08lx -- %#08lx",
> + __entry->start, __entry->end)
> +);
> +
> +TRACE_EVENT(kvm_test_age_hva,
> + TP_PROTO(unsigned long hva),
> + TP_ARGS(hva),
> +
> + TP_STRUCT__entry(
> + __field( unsigned long, hva )
> + ),
> +
> + TP_fast_assign(
> + __entry->hva = hva;
> + ),
> +
> + TP_printk("mmu notifier test age hva: %#08lx", __entry->hva)
> +);
> +
> TRACE_EVENT(kvm_hvc,
> TP_PROTO(unsigned long vcpu_pc, unsigned long r0, unsigned long imm),
> TP_ARGS(vcpu_pc, r0, imm),
> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> index 8afb863..0d738f2 100644
> --- a/arch/arm64/include/asm/kvm_arm.h
> +++ b/arch/arm64/include/asm/kvm_arm.h
> @@ -212,6 +212,7 @@
>
>
> #define FSC_FAULT (0x04)
> +#define FSC_ACCESS (0x08)
> #define FSC_PERM (0x0c)
>
> /* Hyp Prefetch Fault Address Register (HPFAR/HDFAR) */
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index acd101a..b831710 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -173,19 +173,10 @@ int kvm_unmap_hva(struct kvm *kvm, unsigned long hva);
> int kvm_unmap_hva_range(struct kvm *kvm,
> unsigned long start, unsigned long end);
> void kvm_set_spte_hva(struct kvm *kvm, unsigned long hva, pte_t pte);
> +int kvm_age_hva(struct kvm *kvm, unsigned long start, unsigned long end);
> +int kvm_test_age_hva(struct kvm *kvm, unsigned long hva);
>
> /* We do not have shadow page tables, hence the empty hooks */
> -static inline int kvm_age_hva(struct kvm *kvm, unsigned long start,
> - unsigned long end)
> -{
> - return 0;
> -}
> -
> -static inline int kvm_test_age_hva(struct kvm *kvm, unsigned long hva)
> -{
> - return 0;
> -}
> -
> static inline void kvm_arch_mmu_notifier_invalidate_page(struct kvm *kvm,
> unsigned long address)
> {
> --
> 2.1.4
>
Otherwise, this looks good.

Thanks,
-Christoffer