From: Fred Griffoul fgriffo@amazon.co.uk
Optimize L1 MSR bitmap access by replacing map/unmap operations with a persistent gfn_to_pfn_cache. This optimization reduces overhead during L2 VM-entry where nested_vmx_prepare_msr_bitmap() merges L1's MSR intercepts with L0's requirements.
Current implementation using kvm_vcpu_map_readonly() and kvm_vcpu_unmap() creates significant performance impact, particularly with unmanaged guest memory.
New implementation: - Initializes a pfn cache when entering VMX operation. - Maintains persistent access throughout operation. - Deactivates cache when VMX operation ends.
Signed-off-by: Fred Griffoul fgriffo@amazon.co.uk --- arch/x86/kvm/vmx/nested.c | 42 +++++++++++++++++++++++++++++++++++---- arch/x86/kvm/vmx/vmx.h | 2 ++ 2 files changed, 40 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index b8ea1969113d..aa4fe1fe571d 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -315,6 +315,34 @@ static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct loaded_vmcs *vmcs) vcpu->arch.regs_dirty = 0; }
+/* + * Maps a single guest page starting at @gpa and lock the cache for access. + */ +static int nested_gpc_lock(struct gfn_to_pfn_cache *gpc, gpa_t gpa) +{ + int err; + + if (WARN_ON_ONCE(!PAGE_ALIGNED(gpa))) + return -EINVAL; +retry: + read_lock(&gpc->lock); + if (!kvm_gpc_check(gpc, PAGE_SIZE) || (gpc->gpa != gpa)) { + read_unlock(&gpc->lock); + err = kvm_gpc_activate(gpc, gpa, PAGE_SIZE); + if (err) + return err; + + goto retry; + } + + return 0; +} + +static void nested_gpc_unlock(struct gfn_to_pfn_cache *gpc) +{ + read_unlock(&gpc->lock); +} + static void nested_put_vmcs12_pages(struct kvm_vcpu *vcpu) { struct vcpu_vmx *vmx = to_vmx(vcpu); @@ -344,6 +372,9 @@ static void free_nested(struct kvm_vcpu *vcpu) vmx->nested.vmxon = false; vmx->nested.smm.vmxon = false; vmx->nested.vmxon_ptr = INVALID_GPA; + + kvm_gpc_deactivate(&vmx->nested.msr_bitmap_cache); + free_vpid(vmx->nested.vpid02); vmx->nested.posted_intr_nv = -1; vmx->nested.current_vmptr = INVALID_GPA; @@ -625,7 +656,7 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu, int msr; unsigned long *msr_bitmap_l1; unsigned long *msr_bitmap_l0 = vmx->nested.vmcs02.msr_bitmap; - struct kvm_host_map map; + struct gfn_to_pfn_cache *gpc;
/* Nothing to do if the MSR bitmap is not in use. */ if (!cpu_has_vmx_msr_bitmap() || @@ -648,10 +679,11 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu, return true; }
- if (kvm_vcpu_map_readonly(vcpu, gpa_to_gfn(vmcs12->msr_bitmap), &map)) + gpc = &vmx->nested.msr_bitmap_cache; + if (nested_gpc_lock(gpc, vmcs12->msr_bitmap)) return false;
- msr_bitmap_l1 = (unsigned long *)map.hva; + msr_bitmap_l1 = (unsigned long *)gpc->khva;
/* * To keep the control flow simple, pay eight 8-byte writes (sixteen @@ -721,7 +753,7 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu, nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0, MSR_IA32_MPERF, MSR_TYPE_R);
- kvm_vcpu_unmap(vcpu, &map); + nested_gpc_unlock(gpc);
vmx->nested.force_msr_bitmap_recalc = false;
@@ -5352,6 +5384,8 @@ static int enter_vmx_operation(struct kvm_vcpu *vcpu)
vmx->nested.vpid02 = allocate_vpid();
+ kvm_gpc_init(&vmx->nested.msr_bitmap_cache, vcpu->kvm); + vmx->nested.vmcs02_initialized = false; vmx->nested.vmxon = true;
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h index d3389baf3ab3..3a6983222841 100644 --- a/arch/x86/kvm/vmx/vmx.h +++ b/arch/x86/kvm/vmx/vmx.h @@ -152,6 +152,8 @@ struct nested_vmx {
struct loaded_vmcs vmcs02;
+ struct gfn_to_pfn_cache msr_bitmap_cache; + /* * Guest pages referred to in the vmcs02 with host-physical * pointers, so we must keep them pinned while L2 runs.