Re: [Patch 2/6] KVM: VMX: Use wrmsr for switching between guest and host IA32_XSS

Paolo Bonzini
2019-10-09T06:32:57Z

On 09/10/19 02:41, Aaron Lewis wrote:
> Set IA32_XSS for the guest and host during VM Enter and VM Exit
> transitions rather than by using the MSR-load areas.
>
> Reviewed-by: Jim Mattson <jmattson@google.com>
> Signed-off-by: Aaron Lewis <aaronlewis@google.com>

This commit message is missing an explanation of why this is a good thing.

Also, the series is missing a cover letter that explains a bit more of
the overall picture. I have no problem with no cover letter for
two-patch series, but at six it is definitely a requirement.

So I'm replying to this patch as a proxy for the whole series, and
asking: why is it useful to enable XSAVES (on AMD or anywhere) if anyway
IA32_XSS is limited to zero?

On AMD, we do have the problem that XSAVES is essentially a WRMSR with
no exit, albeit confined to the MSRs included in the set bits of
IA32_XSS. But while that would be a (good) argument for writing 0 to
IA32_XSS around AMD vmentry, it shouldn't require making XSAVES
available to the guests.

Thanks,

Paolo

> ---
> arch/x86/kvm/svm.c | 4 ++--
> arch/x86/kvm/vmx/vmx.c | 14 ++------------
> arch/x86/kvm/x86.c | 25 +++++++++++++++++++++----
> arch/x86/kvm/x86.h | 4 ++--
> 4 files changed, 27 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index f8ecb6df5106..e2d7a7738c76 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -5628,7 +5628,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> svm->vmcb->save.cr2 = vcpu->arch.cr2;
>
> clgi();
> - kvm_load_guest_xcr0(vcpu);
> + kvm_load_guest_xsave_controls(vcpu);
>
> if (lapic_in_kernel(vcpu) &&
> vcpu->arch.apic->lapic_timer.timer_advance_ns)
> @@ -5778,7 +5778,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
> kvm_before_interrupt(&svm->vcpu);
>
> - kvm_put_guest_xcr0(vcpu);
> + kvm_load_host_xsave_controls(vcpu);
> stgi();
>
> /* Any pending NMI will happen here */
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 409e9a7323f1..ff5ba28abecb 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -106,8 +106,6 @@ module_param(enable_apicv, bool, S_IRUGO);
> static bool __read_mostly nested = 1;
> module_param(nested, bool, S_IRUGO);
>
> -static u64 __read_mostly host_xss;
> -
> bool __read_mostly enable_pml = 1;
> module_param_named(pml, enable_pml, bool, S_IRUGO);
>
> @@ -2074,11 +2072,6 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> if (data != 0)
> return 1;
> vcpu->arch.ia32_xss = data;
> - if (vcpu->arch.ia32_xss != host_xss)
> - add_atomic_switch_msr(vmx, MSR_IA32_XSS,
> - vcpu->arch.ia32_xss, host_xss, false);
> - else
> - clear_atomic_switch_msr(vmx, MSR_IA32_XSS);
> break;
> case MSR_IA32_RTIT_CTL:
> if ((pt_mode != PT_MODE_HOST_GUEST) ||
> @@ -6540,7 +6533,7 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
> vmx_set_interrupt_shadow(vcpu, 0);
>
> - kvm_load_guest_xcr0(vcpu);
> + kvm_load_guest_xsave_controls(vcpu);
>
> if (static_cpu_has(X86_FEATURE_PKU) &&
> kvm_read_cr4_bits(vcpu, X86_CR4_PKE) &&
> @@ -6647,7 +6640,7 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> __write_pkru(vmx->host_pkru);
> }
>
> - kvm_put_guest_xcr0(vcpu);
> + kvm_load_host_xsave_controls(vcpu);
>
> vmx->nested.nested_run_pending = 0;
> vmx->idt_vectoring_info = 0;
> @@ -7599,9 +7592,6 @@ static __init int hardware_setup(void)
> WARN_ONCE(host_bndcfgs, "KVM: BNDCFGS in host will be lost");
> }
>
> - if (boot_cpu_has(X86_FEATURE_XSAVES))
> - rdmsrl(MSR_IA32_XSS, host_xss);
> -
> if (!cpu_has_vmx_vpid() || !cpu_has_vmx_invvpid() ||
> !(cpu_has_vmx_invvpid_single() || cpu_has_vmx_invvpid_global()))
> enable_vpid = 0;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 661e2bf38526..e90e658fd8a9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -176,6 +176,8 @@ struct kvm_shared_msrs {
> static struct kvm_shared_msrs_global __read_mostly shared_msrs_global;
> static struct kvm_shared_msrs __percpu *shared_msrs;
>
> +static u64 __read_mostly host_xss;
> +
> struct kvm_stats_debugfs_item debugfs_entries[] = {
> { "pf_fixed", VCPU_STAT(pf_fixed) },
> { "pf_guest", VCPU_STAT(pf_guest) },
> @@ -812,27 +814,39 @@ void kvm_lmsw(struct kvm_vcpu *vcpu, unsigned long msw)
> }
> EXPORT_SYMBOL_GPL(kvm_lmsw);
>
> -void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu)
> +void kvm_load_guest_xsave_controls(struct kvm_vcpu *vcpu)
> {
> if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE) &&
> !vcpu->guest_xcr0_loaded) {
> /* kvm_set_xcr() also depends on this */
> if (vcpu->arch.xcr0 != host_xcr0)
> xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
> +
> + if (kvm_x86_ops->xsaves_supported() &&
> + guest_cpuid_has(vcpu, X86_FEATURE_XSAVES) &&
> + vcpu->arch.ia32_xss != host_xss)
> + wrmsrl(MSR_IA32_XSS, vcpu->arch.ia32_xss);
> +
> vcpu->guest_xcr0_loaded = 1;
> }
> }
> -EXPORT_SYMBOL_GPL(kvm_load_guest_xcr0);
> +EXPORT_SYMBOL_GPL(kvm_load_guest_xsave_controls);
>
> -void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu)
> +void kvm_load_host_xsave_controls(struct kvm_vcpu *vcpu)
> {
> if (vcpu->guest_xcr0_loaded) {
> if (vcpu->arch.xcr0 != host_xcr0)
> xsetbv(XCR_XFEATURE_ENABLED_MASK, host_xcr0);
> +
> + if (kvm_x86_ops->xsaves_supported() &&
> + guest_cpuid_has(vcpu, X86_FEATURE_XSAVES) &&
> + vcpu->arch.ia32_xss != host_xss)
> + wrmsrl(MSR_IA32_XSS, host_xss);
> +
> vcpu->guest_xcr0_loaded = 0;
> }
> }
> -EXPORT_SYMBOL_GPL(kvm_put_guest_xcr0);
> +EXPORT_SYMBOL_GPL(kvm_load_host_xsave_controls);
>
> static int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
> {
> @@ -9293,6 +9307,9 @@ int kvm_arch_hardware_setup(void)
> kvm_default_tsc_scaling_ratio = 1ULL << kvm_tsc_scaling_ratio_frac_bits;
> }
>
> + if (boot_cpu_has(X86_FEATURE_XSAVES))
> + rdmsrl(MSR_IA32_XSS, host_xss);
> +
> kvm_init_msr_list();
> return 0;
> }
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index dbf7442a822b..0d04e865665b 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -366,7 +366,7 @@ static inline bool kvm_pat_valid(u64 data)
> return (data | ((data & 0x0202020202020202ull) << 1)) == data;
> }
>
> -void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu);
> -void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu);
> +void kvm_load_guest_xsave_controls(struct kvm_vcpu *vcpu);
> +void kvm_load_host_xsave_controls(struct kvm_vcpu *vcpu);
>
> #endif
>