[PATCH v4 07/10] KVM: x86/MMU: Allow NX huge pages to be disabled on a per-vm basis
From: Ben Gardon
Date: Mon Apr 11 2022 - 17:11:08 EST
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.
Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>
Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
---
Documentation/virt/kvm/api.rst | 11 +++++++++++
arch/x86/include/asm/kvm_host.h | 2 ++
arch/x86/kvm/mmu.h | 10 ++++++----
arch/x86/kvm/mmu/mmu.c | 2 +-
arch/x86/kvm/mmu/spte.c | 7 ++++---
arch/x86/kvm/mmu/spte.h | 3 ++-
arch/x86/kvm/mmu/tdp_mmu.c | 3 ++-
arch/x86/kvm/x86.c | 6 ++++++
include/uapi/linux/kvm.h | 1 +
9 files changed, 35 insertions(+), 10 deletions(-)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 72183ae628f7..31fb002632bb 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7855,6 +7855,17 @@ At this time, KVM_PMU_CAP_DISABLE is the only capability. Setting
this capability will disable PMU virtualization for that VM. Usermode
should adjust CPUID leaf 0xA to reflect that the PMU is disabled.
+8.36 KVM_CAP_VM_DISABLE_NX_HUGE_PAGES
+---------------------------
+
+:Capability KVM_CAP_PMU_CAPABILITY
+:Architectures: x86
+:Type: vm
+
+This capability disables the NX huge pages mitigation for iTLB MULTIHIT.
+
+The capability has no effect if the nx_huge_pages module parameter is not set.
+
9. Known KVM API problems
=========================
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 2c20f715f009..b8ab4fa7d4b2 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1240,6 +1240,8 @@ struct kvm_arch {
hpa_t hv_root_tdp;
spinlock_t hv_root_tdp_lock;
#endif
+
+ bool disable_nx_huge_pages;
};
struct kvm_vm_stat {
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 671cfeccf04e..20d12e5b0040 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -173,10 +173,12 @@ struct kvm_page_fault {
int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault);
extern int nx_huge_pages;
-static inline bool is_nx_huge_page_enabled(void)
+static inline bool is_nx_huge_page_enabled(struct kvm *kvm)
{
- return READ_ONCE(nx_huge_pages);
+ return READ_ONCE(nx_huge_pages) &&
+ !kvm->arch.disable_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)
@@ -191,8 +193,8 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
.user = err & PFERR_USER_MASK,
.prefetch = prefetch,
.is_tdp = likely(vcpu->arch.mmu->page_fault == kvm_tdp_page_fault),
- .nx_huge_page_workaround_enabled = is_nx_huge_page_enabled(),
-
+ .nx_huge_page_workaround_enabled =
+ is_nx_huge_page_enabled(vcpu->kvm),
.max_level = KVM_MAX_HUGEPAGE_LEVEL,
.req_level = PG_LEVEL_4K,
.goal_level = PG_LEVEL_4K,
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index caaa610b7878..149f353105f4 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6144,7 +6144,7 @@ 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)
{
mutex_lock(&kvm->slots_lock);
kvm_mmu_zap_all_fast(kvm);
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 4739b53c9734..877ad30bc7ad 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -116,7 +116,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
spte |= spte_shadow_accessed_mask(spte);
if (level > PG_LEVEL_4K && (pte_access & ACC_EXEC_MASK) &&
- is_nx_huge_page_enabled()) {
+ is_nx_huge_page_enabled(vcpu->kvm)) {
pte_access &= ~ACC_EXEC_MASK;
}
@@ -215,7 +215,8 @@ static u64 make_spte_executable(u64 spte)
* This is used during huge page splitting to build the SPTEs that make up the
* new page table.
*/
-u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index)
+u64 make_huge_page_split_spte(struct kvm *kvm, u64 huge_spte, int huge_level,
+ int index)
{
u64 child_spte;
int child_level;
@@ -243,7 +244,7 @@ u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index)
* When splitting to a 4K page, mark the page executable as the
* NX hugepage mitigation no longer applies.
*/
- if (is_nx_huge_page_enabled())
+ if (is_nx_huge_page_enabled(kvm))
child_spte = make_spte_executable(child_spte);
}
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 73f12615416f..e4142caff4b1 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -415,7 +415,8 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,
u64 old_spte, bool prefetch, bool can_unsync,
bool host_writable, u64 *new_spte);
-u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index);
+u64 make_huge_page_split_spte(struct kvm *kvm, u64 huge_spte, int huge_level,
+ int index);
u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled);
u64 make_mmio_spte(struct kvm_vcpu *vcpu, u64 gfn, unsigned int access);
u64 mark_spte_for_access_track(u64 spte);
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 566548a3efa7..03aa1e0f60e2 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1469,7 +1469,8 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
* not been linked in yet and thus is not reachable from any other CPU.
*/
for (i = 0; i < PT64_ENT_PER_PAGE; i++)
- sp->spt[i] = make_huge_page_split_spte(huge_spte, level, i);
+ sp->spt[i] = make_huge_page_split_spte(kvm, huge_spte,
+ level, i);
/*
* Replace the huge spte with a pointer to the populated lower level
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ab336f7c82e4..b810ea45f965 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4286,6 +4286,7 @@ 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:
r = 1;
break;
case KVM_CAP_EXIT_HYPERCALL:
@@ -6079,6 +6080,11 @@ 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 dd1d8167e71f..7155488164bd 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1148,6 +1148,7 @@ struct kvm_ppc_resize_hpt {
#define KVM_CAP_PMU_CAPABILITY 212
#define KVM_CAP_DISABLE_QUIRKS2 213
#define KVM_CAP_VM_TSC_CONTROL 214
+#define KVM_CAP_VM_DISABLE_NX_HUGE_PAGES 215
#ifdef KVM_CAP_IRQ_ROUTING
--
2.35.1.1178.g4f1659d476-goog