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Date: Wed, 16 Aug 2023 11:21:23 +0800
From: Yuan Yao <yuan.yao@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, Zeng Guang <guang.zeng@...el.com>,
Yuan Yao <yuan.yao@...el.com>
Subject: Re: [PATCH v3 06/15] KVM: x86: Use KVM-governed feature framework to
track "XSAVES enabled"
On Wed, Aug 16, 2023 at 10:58:41AM +0800, Yuan Yao wrote:
> On Tue, Aug 15, 2023 at 01:36:44PM -0700, Sean Christopherson wrote:
> > Use the governed feature framework to track if XSAVES is "enabled", i.e.
> > if XSAVES can be used by the guest. Add a comment in the SVM code to
> > explain the very unintuitive logic of deliberately NOT checking if XSAVES
> > is enumerated in the guest CPUID model.
> >
> > No functional change intended.
> >
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> > arch/x86/include/asm/kvm_host.h | 1 -
> > arch/x86/kvm/governed_features.h | 1 +
> > arch/x86/kvm/svm/svm.c | 17 ++++++++++++---
> > arch/x86/kvm/vmx/vmx.c | 36 ++++++++++++++++----------------
> > arch/x86/kvm/x86.c | 4 ++--
> > 5 files changed, 35 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 60d430b4650f..9f57aa33798b 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -746,7 +746,6 @@ struct kvm_vcpu_arch {
> > u64 smi_count;
> > bool at_instruction_boundary;
> > bool tpr_access_reporting;
> > - bool xsaves_enabled;
> > bool xfd_no_write_intercept;
> > u64 ia32_xss;
> > u64 microcode_version;
> > diff --git a/arch/x86/kvm/governed_features.h b/arch/x86/kvm/governed_features.h
> > index b29c15d5e038..b896a64e4ac3 100644
> > --- a/arch/x86/kvm/governed_features.h
> > +++ b/arch/x86/kvm/governed_features.h
> > @@ -6,6 +6,7 @@ BUILD_BUG()
> > #define KVM_GOVERNED_X86_FEATURE(x) KVM_GOVERNED_FEATURE(X86_FEATURE_##x)
> >
> > KVM_GOVERNED_X86_FEATURE(GBPAGES)
> > +KVM_GOVERNED_X86_FEATURE(XSAVES)
> >
> > #undef KVM_GOVERNED_X86_FEATURE
> > #undef KVM_GOVERNED_FEATURE
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 6aaa3c7b4578..d67f6e23dcd2 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -4273,9 +4273,20 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > struct vcpu_svm *svm = to_svm(vcpu);
> > struct kvm_cpuid_entry2 *best;
> >
> > - vcpu->arch.xsaves_enabled = guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
> > - boot_cpu_has(X86_FEATURE_XSAVE) &&
> > - boot_cpu_has(X86_FEATURE_XSAVES);
> > + /*
> > + * SVM doesn't provide a way to disable just XSAVES in the guest, KVM
> > + * can only disable all variants of by disallowing CR4.OSXSAVE from
> > + * being set. As a result, if the host has XSAVE and XSAVES, and the
> > + * guest has XSAVE enabled, the guest can execute XSAVES without
> > + * faulting. Treat XSAVES as enabled in this case regardless of
> > + * whether it's advertised to the guest so that KVM context switches
> > + * XSS on VM-Enter/VM-Exit. Failure to do so would effectively give
> > + * the guest read/write access to the host's XSS.
> > + */
> > + if (boot_cpu_has(X86_FEATURE_XSAVE) &&
> > + boot_cpu_has(X86_FEATURE_XSAVES) &&
> > + guest_cpuid_has(vcpu, X86_FEATURE_XSAVE))
> > + kvm_governed_feature_set(vcpu, X86_FEATURE_XSAVES);
> >
> > /* Update nrips enabled cache */
> > svm->nrips_enabled = kvm_cpu_cap_has(X86_FEATURE_NRIPS) &&
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 22975cc949b7..6314ca32a5cf 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -4543,16 +4543,19 @@ vmx_adjust_secondary_exec_control(struct vcpu_vmx *vmx, u32 *exec_control,
> > * based on a single guest CPUID bit, with a dedicated feature bit. This also
> > * verifies that the control is actually supported by KVM and hardware.
> > */
> > -#define vmx_adjust_sec_exec_control(vmx, exec_control, name, feat_name, ctrl_name, exiting) \
> > -({ \
> > - bool __enabled; \
> > - \
> > - if (cpu_has_vmx_##name()) { \
> > - __enabled = guest_cpuid_has(&(vmx)->vcpu, \
> > - X86_FEATURE_##feat_name); \
> > - vmx_adjust_secondary_exec_control(vmx, exec_control, \
> > - SECONDARY_EXEC_##ctrl_name, __enabled, exiting); \
> > - } \
> > +#define vmx_adjust_sec_exec_control(vmx, exec_control, name, feat_name, ctrl_name, exiting) \
> > +({ \
> > + struct kvm_vcpu *__vcpu = &(vmx)->vcpu; \
> > + bool __enabled; \
> > + \
> > + if (cpu_has_vmx_##name()) { \
> > + if (kvm_is_governed_feature(X86_FEATURE_##feat_name)) \
> > + __enabled = guest_can_use(__vcpu, X86_FEATURE_##feat_name); \
> > + else \
> > + __enabled = guest_cpuid_has(__vcpu, X86_FEATURE_##feat_name); \
> > + vmx_adjust_secondary_exec_control(vmx, exec_control, SECONDARY_EXEC_##ctrl_name,\
> > + __enabled, exiting); \
> > + } \
> > })
> >
> > /* More macro magic for ENABLE_/opt-in versus _EXITING/opt-out controls. */
> > @@ -4612,10 +4615,7 @@ static u32 vmx_secondary_exec_control(struct vcpu_vmx *vmx)
> > if (!enable_pml || !atomic_read(&vcpu->kvm->nr_memslots_dirty_logging))
> > exec_control &= ~SECONDARY_EXEC_ENABLE_PML;
> >
> > - if (cpu_has_vmx_xsaves())
> > - vmx_adjust_secondary_exec_control(vmx, &exec_control,
> > - SECONDARY_EXEC_ENABLE_XSAVES,
> > - vcpu->arch.xsaves_enabled, false);
> > + vmx_adjust_sec_exec_feature(vmx, &exec_control, xsaves, XSAVES);
> >
> > /*
> > * RDPID is also gated by ENABLE_RDTSCP, turn on the control if either
> > @@ -4634,6 +4634,7 @@ static u32 vmx_secondary_exec_control(struct vcpu_vmx *vmx)
> > SECONDARY_EXEC_ENABLE_RDTSCP,
> > rdpid_or_rdtscp_enabled, false);
> > }
> > +
> > vmx_adjust_sec_exec_feature(vmx, &exec_control, invpcid, INVPCID);
> >
> > vmx_adjust_sec_exec_exiting(vmx, &exec_control, rdrand, RDRAND);
> > @@ -7745,10 +7746,9 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > * to the guest. XSAVES depends on CR4.OSXSAVE, and CR4.OSXSAVE can be
> > * set if and only if XSAVE is supported.
> > */
> > - vcpu->arch.xsaves_enabled = kvm_cpu_cap_has(X86_FEATURE_XSAVES) &&
> > - boot_cpu_has(X86_FEATURE_XSAVE) &&
> > - guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
> > - guest_cpuid_has(vcpu, X86_FEATURE_XSAVES);
> > + if (boot_cpu_has(X86_FEATURE_XSAVE) &&
> > + guest_cpuid_has(vcpu, X86_FEATURE_XSAVE))
>
> Should above 2 be X86_FEATURE_XSAVES ? XSAVE and XSAVES have different
> cpuid definition.
> Otherwise X86_FEATURE_XSAVES is allowed in governor even XSAVES
> is not exposed to guest cpuid, with unnecessary context switches.
Oh! false alarm.
I just forgot that kvm_governed_feature_check_and_set() does checks
on kvm cpu cap and guest cpuid set, thus no problem.
Reviewed-by: Yuan Yao <yuan.yao@...el.com>
>
> > + kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_XSAVES);
> >
> > vmx_setup_uret_msrs(vmx);
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index eba35d43e3fe..34945c7dba38 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1016,7 +1016,7 @@ void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
> > if (vcpu->arch.xcr0 != host_xcr0)
> > xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
> >
> > - if (vcpu->arch.xsaves_enabled &&
> > + if (guest_can_use(vcpu, X86_FEATURE_XSAVES) &&
> > vcpu->arch.ia32_xss != host_xss)
> > wrmsrl(MSR_IA32_XSS, vcpu->arch.ia32_xss);
> > }
> > @@ -1047,7 +1047,7 @@ void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
> > if (vcpu->arch.xcr0 != host_xcr0)
> > xsetbv(XCR_XFEATURE_ENABLED_MASK, host_xcr0);
> >
> > - if (vcpu->arch.xsaves_enabled &&
> > + if (guest_can_use(vcpu, X86_FEATURE_XSAVES) &&
> > vcpu->arch.ia32_xss != host_xss)
> > wrmsrl(MSR_IA32_XSS, host_xss);
> > }
> > --
> > 2.41.0.694.ge786442a9b-goog
> >
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