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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id nd36sm1151992ejc.21.2021.04.21.23.45.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 21 Apr 2021 23:45:30 -0700 (PDT) Subject: Re: [PATCH v2] KVM: SVM: Delay restoration of host MSR_TSC_AUX until return to userspace To: Sean Christopherson Cc: Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org References: <20210422001736.3255735-1-seanjc@google.com> From: Paolo Bonzini Message-ID: <71038b23-0902-1074-bd94-d5b30311faf0@redhat.com> Date: Thu, 22 Apr 2021 08:45:29 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <20210422001736.3255735-1-seanjc@google.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 22/04/21 02:17, Sean Christopherson wrote: > 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. > > Signed-off-by: Sean Christopherson > --- > > v2: Rebased to kvm/queue (ish), commit 0e91d1992235 ("KVM: SVM: Allocate SEV > command structures on local stack") > > arch/x86/kvm/svm/svm.c | 50 ++++++++++++++++++------------------------ > arch/x86/kvm/svm/svm.h | 7 ------ > 2 files changed, 21 insertions(+), 36 deletions(-) > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c > index cd8c333ed2dc..596361449f25 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; > @@ -1423,19 +1435,10 @@ 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) > @@ -2893,12 +2882,15 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr) > return 1; > > /* > - * 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_acess_msrs, and switch it via user return. > */ > svm->tsc_aux = data; > - wrmsrl(MSR_TSC_AUX, svm->tsc_aux); > + > + preempt_disable(); > + kvm_set_user_return_msr(TSC_AUX_URET_SLOT, data, -1ull); > + preempt_enable(); > break; > case MSR_IA32_DEBUGCTLMSR: > if (!boot_cpu_has(X86_FEATURE_LBRV)) { > diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h > index 454da1c1d9b7..9dce6f290041 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 > > @@ -128,8 +123,6 @@ 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 > Queued, thanks. Paolo