On Fri, 2023-11-10 at 15:55 -0800, Sean Christopherson wrote:
> When updating guest CPUID entries to emulate runtime behavior, e.g. when
> the guest enables a CR4-based feature that is tied to a CPUID flag, also
> update the vCPU's cpu_caps accordingly. This will allow replacing all
> usage of guest_cpuid_has() with guest_cpu_cap_has().
>
> Take care not to update guest capabilities when KVM is updating CPUID
> entries that *may* become the vCPU's CPUID, e.g. if userspace tries to set
> bogus CPUID information. No extra call to update cpu_caps is needed as
> the cpu_caps are initialized from the incoming guest CPUID, i.e. will
> automatically get the updated values.
>
> Note, none of the features in question use guest_cpu_cap_has() at this
> time, i.e. aside from settings bits in cpu_caps, this is a glorified nop.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kvm/cpuid.c | 48 +++++++++++++++++++++++++++++++-------------
> 1 file changed, 34 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 36bd04030989..37a991439fe6 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -262,31 +262,48 @@ static u64 cpuid_get_supported_xcr0(struct kvm_cpuid_entry2 *entries, int nent)
> return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
> }
>
> +static __always_inline void kvm_update_feature_runtime(struct kvm_vcpu *vcpu,
> + struct kvm_cpuid_entry2 *entry,
> + unsigned int x86_feature,
> + bool has_feature)
> +{
> + if (entry)
> + cpuid_entry_change(entry, x86_feature, has_feature);
> +
> + if (vcpu)
> + guest_cpu_cap_change(vcpu, x86_feature, has_feature);
> +}
> +
> static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,
> int nent)
> {
> struct kvm_cpuid_entry2 *best;
> + struct kvm_vcpu *caps = vcpu;
> +
> + /*
> + * Don't update vCPU capabilities if KVM is updating CPUID entries that
> + * are coming in from userspace!
> + */
> + if (entries != vcpu->arch.cpuid_entries)
> + caps = NULL;
I think that this should be decided by the caller. Just a boolean will suffice.
Or even better: since the userspace CPUID update is really not important in terms of performance,
why to special case it?
Even if these guest caps are later overwritten, I don't see why we
need to avoid updating them, and in fact introduce a small risk of them not being consistent
with the other cpu caps.
With this we can avoid having the 'cap' variable which is *very* confusing as well.
>
> best = cpuid_entry2_find(entries, nent, 1, KVM_CPUID_INDEX_NOT_SIGNIFICANT);
> - if (best) {
> - /* Update OSXSAVE bit */
> - if (boot_cpu_has(X86_FEATURE_XSAVE))
> - cpuid_entry_change(best, X86_FEATURE_OSXSAVE,
> +
> + if (boot_cpu_has(X86_FEATURE_XSAVE))
> + kvm_update_feature_runtime(caps, best, X86_FEATURE_OSXSAVE,
> kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE));
>
> - cpuid_entry_change(best, X86_FEATURE_APIC,
> - vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
> + kvm_update_feature_runtime(caps, best, X86_FEATURE_APIC,
> + vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
>
> - if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT))
> - cpuid_entry_change(best, X86_FEATURE_MWAIT,
> - vcpu->arch.ia32_misc_enable_msr &
> - MSR_IA32_MISC_ENABLE_MWAIT);
> - }
> + if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT))
> + kvm_update_feature_runtime(caps, best, X86_FEATURE_MWAIT,
> + vcpu->arch.ia32_misc_enable_msr & MSR_IA32_MISC_ENABLE_MWAIT);
>
> best = cpuid_entry2_find(entries, nent, 7, 0);
> - if (best && boot_cpu_has(X86_FEATURE_PKU))
> - cpuid_entry_change(best, X86_FEATURE_OSPKE,
> - kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE));
> + if (boot_cpu_has(X86_FEATURE_PKU))
> + kvm_update_feature_runtime(caps, best, X86_FEATURE_OSPKE,
> + kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE));
>
> best = cpuid_entry2_find(entries, nent, 0xD, 0);
> if (best)
> @@ -353,6 +370,9 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> * Reset guest capabilities to userspace's guest CPUID definition, i.e.
> * honor userspace's definition for features that don't require KVM or
> * hardware management/support (or that KVM simply doesn't care about).
> + *
> + * Note, KVM has already done runtime updates on guest CPUID, i.e. this
> + * will also correctly set runtime features in guest CPU capabilities.
> */
> for (i = 0; i < NR_KVM_CPU_CAPS; i++) {
> const struct cpuid_reg cpuid = reverse_cpuid[i];
Best regards,
Maxim Levitsky
