On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang wrote:
> Update CPUID.(EAX=0DH,ECX=1).EBX to reflect current required xstate size
> due to XSS MSR modification.
> CPUID(EAX=0DH,ECX=1).EBX reports the required storage size of all enabled
> xstate features in (XCR0 | IA32_XSS). The CPUID value can be used by guest
> before allocate sufficient xsave buffer.
>
> Note, KVM does not yet support any XSS based features, i.e. supported_xss
> is guaranteed to be zero at this time.
>
> Opportunistically modify XSS write access logic as:
> If XSAVES is not enabled in the guest CPUID, forbid setting IA32_XSS msr
> to anything but 0, even if the write is host initiated.
>
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Co-developed-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 3 ++-
> arch/x86/kvm/cpuid.c | 15 ++++++++++++++-
> arch/x86/kvm/x86.c | 16 ++++++++++++----
> 3 files changed, 28 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 499bd42e3a32..f536102f1eca 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -756,7 +756,6 @@ struct kvm_vcpu_arch {
> bool at_instruction_boundary;
> bool tpr_access_reporting;
> bool xfd_no_write_intercept;
> - u64 ia32_xss;
> u64 microcode_version;
> u64 arch_capabilities;
> u64 perf_capabilities;
> @@ -812,6 +811,8 @@ struct kvm_vcpu_arch {
>
> u64 xcr0;
> u64 guest_supported_xcr0;
> + u64 guest_supported_xss;
> + u64 ia32_xss;
>
> struct kvm_pio_request pio;
> void *pio_data;
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0351e311168a..1d9843b34196 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -275,7 +275,8 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
> best = cpuid_entry2_find(entries, nent, 0xD, 1);
> if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
> cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
> - best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> + best->ebx = xstate_required_size(vcpu->arch.xcr0 |
> + vcpu->arch.ia32_xss, true);
>
> best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
> if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> @@ -312,6 +313,17 @@ static u64 vcpu_get_supported_xcr0(struct kvm_vcpu *vcpu)
> return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
> }
>
> +static u64 vcpu_get_supported_xss(struct kvm_vcpu *vcpu)
> +{
> + struct kvm_cpuid_entry2 *best;
> +
> + best = kvm_find_cpuid_entry_index(vcpu, 0xd, 1);
> + if (!best)
> + return 0;
> +
> + return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
> +}
> +
> static bool kvm_cpuid_has_hyperv(struct kvm_cpuid_entry2 *entries, int nent)
> {
> struct kvm_cpuid_entry2 *entry;
> @@ -358,6 +370,7 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> }
>
> vcpu->arch.guest_supported_xcr0 = vcpu_get_supported_xcr0(vcpu);
> + vcpu->arch.guest_supported_xss = vcpu_get_supported_xss(vcpu);
>
> kvm_update_pv_runtime(vcpu);
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index f7d4cc61bc55..649a100ffd25 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3901,20 +3901,28 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> vcpu->arch.ia32_tsc_adjust_msr += adj;
> }
> break;
> - case MSR_IA32_XSS:
> - if (!msr_info->host_initiated &&
> - !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
> + case MSR_IA32_XSS: {
> + /*
> + * If KVM reported support of XSS MSR, even guest CPUID doesn't
> + * support XSAVES, still allow userspace to set default value(0)
> + * to this MSR.
> + */
> + if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES) &&
> + !(msr_info->host_initiated && data == 0))
> return 1;
> /*
> * KVM supports exposing PT to the guest, but does not support
> * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
> * XSAVES/XRSTORS to save/restore PT MSRs.
> */
> - if (data & ~kvm_caps.supported_xss)
> + if (data & ~vcpu->arch.guest_supported_xss)
> return 1;
> + if (vcpu->arch.ia32_xss == data)
> + break;
> vcpu->arch.ia32_xss = data;
> kvm_update_cpuid_runtime(vcpu);
> break;
> + }
> case MSR_SMI_COUNT:
> if (!msr_info->host_initiated)
> return 1;
Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Best regards,
Maxim Levitsky
