From 844d69c26d836fde97bf6f38a0a69da9fa42e985 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 23 Apr 2021 15:34:04 -0700 Subject: [PATCH] KVM: SVM: Delay restoration of host MSR_TSC_AUX until return to userspace Use KVM's "user return MSRs" framework to defer restoring the host's MSR_TSC_AUX until the CPU returns to userspace. Add/improve comments to clarify why MSR_TSC_AUX is intercepted on both RDMSR and WRMSR, and why it's safe for KVM to keep the guest's value loaded even if KVM is scheduled out. Cc: Reiji Watanabe Signed-off-by: Sean Christopherson Message-Id: <20210423223404.3860547-5-seanjc@google.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/svm/svm.c | 53 +++++++++++++++++++++++--------------------------- arch/x86/kvm/svm/svm.h | 7 ------- 2 files changed, 24 insertions(+), 36 deletions(-) diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 9238831..3017925 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -213,6 +213,15 @@ struct kvm_ldttss_desc { DEFINE_PER_CPU(struct svm_cpu_data *, svm_data); +/* + * Only MSR_TSC_AUX is switched via the user return hook. EFER is switched via + * the VMCB, and the SYSCALL/SYSENTER MSRs are handled by VMLOAD/VMSAVE. + * + * RDTSCP and RDPID are not used in the kernel, specifically to allow KVM to + * defer the restoration of TSC_AUX until the CPU returns to userspace. + */ +#define TSC_AUX_URET_SLOT 0 + static const u32 msrpm_ranges[] = {0, 0xc0000000, 0xc0010000}; #define NUM_MSR_MAPS ARRAY_SIZE(msrpm_ranges) @@ -958,6 +967,9 @@ static __init int svm_hardware_setup(void) kvm_tsc_scaling_ratio_frac_bits = 32; } + if (boot_cpu_has(X86_FEATURE_RDTSCP)) + kvm_define_user_return_msr(TSC_AUX_URET_SLOT, MSR_TSC_AUX); + /* Check for pause filtering support */ if (!boot_cpu_has(X86_FEATURE_PAUSEFILTER)) { pause_filter_count = 0; @@ -1423,20 +1435,11 @@ static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu) { struct vcpu_svm *svm = to_svm(vcpu); struct svm_cpu_data *sd = per_cpu(svm_data, vcpu->cpu); - unsigned int i; if (svm->guest_state_loaded) return; /* - * Certain MSRs are restored on VMEXIT (sev-es), or vmload of host save - * area (non-sev-es). Save ones that aren't so we can restore them - * individually later. - */ - for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++) - rdmsrl(host_save_user_msrs[i], svm->host_user_msrs[i]); - - /* * Save additional host state that will be restored on VMEXIT (sev-es) * or subsequent vmload of host save area. */ @@ -1454,29 +1457,15 @@ static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu) } } - /* This assumes that the kernel never uses MSR_TSC_AUX */ if (static_cpu_has(X86_FEATURE_RDTSCP)) - wrmsrl(MSR_TSC_AUX, svm->tsc_aux); + kvm_set_user_return_msr(TSC_AUX_URET_SLOT, svm->tsc_aux, -1ull); svm->guest_state_loaded = true; } static void svm_prepare_host_switch(struct kvm_vcpu *vcpu) { - struct vcpu_svm *svm = to_svm(vcpu); - unsigned int i; - - if (!svm->guest_state_loaded) - return; - - /* - * Certain MSRs are restored on VMEXIT (sev-es), or vmload of host save - * area (non-sev-es). Restore the ones that weren't. - */ - for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++) - wrmsrl(host_save_user_msrs[i], svm->host_user_msrs[i]); - - svm->guest_state_loaded = false; + to_svm(vcpu)->guest_state_loaded = false; } static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu) @@ -2790,6 +2779,7 @@ static int svm_set_vm_cr(struct kvm_vcpu *vcpu, u64 data) static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr) { struct vcpu_svm *svm = to_svm(vcpu); + int r; u32 ecx = msr->index; u64 data = msr->data; @@ -2910,11 +2900,16 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr) data = (u32)data; /* - * This is rare, so we update the MSR here instead of using - * direct_access_msrs. Doing that would require a rdmsr in - * svm_vcpu_put. + * TSC_AUX is usually changed only during boot and never read + * directly. Intercept TSC_AUX instead of exposing it to the + * guest via direct_access_msrs, and switch it via user return. */ - wrmsrl(MSR_TSC_AUX, data); + preempt_disable(); + r = kvm_set_user_return_msr(TSC_AUX_URET_SLOT, data, -1ull); + preempt_enable(); + if (r) + return 1; + svm->tsc_aux = data; break; case MSR_IA32_DEBUGCTLMSR: diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h index d620619..5d8027e 100644 --- a/arch/x86/kvm/svm/svm.h +++ b/arch/x86/kvm/svm/svm.h @@ -23,11 +23,6 @@ #define __sme_page_pa(x) __sme_set(page_to_pfn(x) << PAGE_SHIFT) -static const u32 host_save_user_msrs[] = { - MSR_TSC_AUX, -}; -#define NR_HOST_SAVE_USER_MSRS ARRAY_SIZE(host_save_user_msrs) - #define IOPM_SIZE PAGE_SIZE * 3 #define MSRPM_SIZE PAGE_SIZE * 2 @@ -129,8 +124,6 @@ struct vcpu_svm { u64 next_rip; - u64 host_user_msrs[NR_HOST_SAVE_USER_MSRS]; - u64 spec_ctrl; /* * Contains guest-controlled bits of VIRT_SPEC_CTRL, which will be -- 2.7.4