On Mon, Mar 21, 2022 at 04:48:42PM -0700, Ben Gardon wrote:
> In some cases, the NX hugepage mitigation for iTLB multihit is not
> needed for all guests on a host. Allow disabling the mitigation on a
> per-VM basis to avoid the performance hit of NX hugepages on trusted
> workloads.
>
> Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/mmu.h | 1 +
> arch/x86/kvm/mmu/mmu.c | 6 ++++--
> arch/x86/kvm/x86.c | 6 ++++++
> include/uapi/linux/kvm.h | 1 +
> 5 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0a0c54639dd8..04ddfc475ce0 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1242,6 +1242,7 @@ struct kvm_arch {
> #endif
>
> bool nx_huge_pages;
> + bool disable_nx_huge_pages;
> };
>
> struct kvm_vm_stat {
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index dd28fe8d13ae..36d8d84ca6c6 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -177,6 +177,7 @@ static inline bool is_nx_huge_page_enabled(struct kvm *kvm)
> {
> return READ_ONCE(kvm->arch.nx_huge_pages);
> }
> +void kvm_update_nx_huge_pages(struct kvm *kvm);
>
> static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> u32 err, bool prefetch)
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index dc9672f70468..a7d387ccfd74 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6195,9 +6195,10 @@ static void __set_nx_huge_pages(bool val)
> nx_huge_pages = itlb_multihit_kvm_mitigation = val;
> }
>
> -static void kvm_update_nx_huge_pages(struct kvm *kvm)
> +void kvm_update_nx_huge_pages(struct kvm *kvm)
> {
> - kvm->arch.nx_huge_pages = nx_huge_pages;
> + kvm->arch.nx_huge_pages = nx_huge_pages &&
> + !kvm->arch.disable_nx_huge_pages;
kvm->arch.nx_huge_pages seems like it could be dropped and
is_nx_huge_page_enabled() could just check this condition.
>
> mutex_lock(&kvm->slots_lock);
> kvm_mmu_zap_all_fast(kvm);
> @@ -6451,6 +6452,7 @@ int kvm_mmu_post_init_vm(struct kvm *kvm)
> int err;
>
> kvm->arch.nx_huge_pages = READ_ONCE(nx_huge_pages);
> + kvm->arch.disable_nx_huge_pages = false;
I believe this can be omitted since kvm_arch is zero-initialized.
> err = kvm_vm_create_worker_thread(kvm, kvm_nx_lpage_recovery_worker, 0,
> "kvm-nx-lpage-recovery",
> &kvm->arch.nx_lpage_recovery_thread);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 51106d32f04e..73df90a6932b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4256,6 +4256,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_SYS_ATTRIBUTES:
> case KVM_CAP_VAPIC:
> case KVM_CAP_ENABLE_CAP:
> + case KVM_CAP_VM_DISABLE_NX_HUGE_PAGES:
Please document the new capability.
> r = 1;
> break;
> case KVM_CAP_EXIT_HYPERCALL:
> @@ -6048,6 +6049,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> }
> mutex_unlock(&kvm->lock);
> break;
> + case KVM_CAP_VM_DISABLE_NX_HUGE_PAGES:
> + kvm->arch.disable_nx_huge_pages = true;
> + kvm_update_nx_huge_pages(kvm);
> + r = 0;
> + break;
> default:
> r = -EINVAL;
> break;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index ee5cc9e2a837..6f9fa7ecfd1e 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1144,6 +1144,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_S390_MEM_OP_EXTENSION 211
> #define KVM_CAP_PMU_CAPABILITY 212
> #define KVM_CAP_DISABLE_QUIRKS2 213
> +#define KVM_CAP_VM_DISABLE_NX_HUGE_PAGES 214
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> --
> 2.35.1.894.gb6a874cedc-goog
>
