kvm_mmu_reset_context is called on all role changes and right now it calls kvm_mmu_unload. With the legacy MMU this is a relatively cheap operation; the previous PGDs remains in the hash table and is picked up immediately on the next page fault. With the TDP MMU, however, the roots are thrown away for good and a full rebuild of the page tables is necessary, which is many times more expensive. Fortunately, throwing away the roots is not necessary except when the manual says a TLB flush is required: - changing CR0.PG from 1 to 0 (because it flushes the TLB according to the x86 architecture specification) - changing CPUID (which changes the interpretation of page tables in ways not reflected by the role). - changing CR4.SMEP from 0 to 1 (not doing so actually breaks access.c) Except for these cases, once the MMU has updated the CPU/MMU roles and metadata it is enough to force-reload the current value of CR3. KVM will look up the cached roots for an entry with the right role and PGD, and only if the cache misses a new root will be created. Measuring with vmexit.flat from kvm-unit-tests shows the following improvement: TDP legacy shadow before 46754 5096 5150 after 4879 4875 5006 which is for very small page tables. The impact is however much larger when running as an L1 hypervisor, because the new page tables cause extra work for L0 to shadow them. Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> --- arch/x86/kvm/mmu/mmu.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index c44b5114f947..913cc7229bf4 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -5043,8 +5043,8 @@ EXPORT_SYMBOL_GPL(kvm_init_mmu); void kvm_mmu_after_set_cpuid(struct kvm_vcpu *vcpu) { /* - * Invalidate all MMU roles to force them to reinitialize as CPUID - * information is factored into reserved bit calculations. + * Invalidate all MMU roles and roots to force them to reinitialize, + * as CPUID information is factored into reserved bit calculations. * * Correctly handling multiple vCPU models with respect to paging and * physical address properties) in a single VM would require tracking @@ -5057,6 +5057,7 @@ void kvm_mmu_after_set_cpuid(struct kvm_vcpu *vcpu) vcpu->arch.root_mmu.mmu_role.ext.valid = 0; vcpu->arch.guest_mmu.mmu_role.ext.valid = 0; vcpu->arch.nested_mmu.mmu_role.ext.valid = 0; + kvm_mmu_unload(vcpu); kvm_mmu_reset_context(vcpu); /* @@ -5068,8 +5069,8 @@ void kvm_mmu_after_set_cpuid(struct kvm_vcpu *vcpu) void kvm_mmu_reset_context(struct kvm_vcpu *vcpu) { - kvm_mmu_unload(vcpu); kvm_init_mmu(vcpu); + kvm_make_request(KVM_REQ_MMU_UPDATE_ROOT, vcpu); } EXPORT_SYMBOL_GPL(kvm_mmu_reset_context); -- 2.31.1