Re: [PATCH v5 04/19] KVM:x86: Refresh CPUID on write to guest MSR_IA32_XSS

Yang, Weijiang
2023-08-08T17:28:56Z

On 8/5/2023 12:02 AM, Sean Christopherson wrote:
> On Thu, Aug 03, 2023, Yang Weijiang wrote:
>> Update CPUID(EAX=0DH,ECX=1) when the guest's XSS is modified.
>> CPUID(EAX=0DH,ECX=1).EBX reports required storage size of
>> all enabled xstate features in XCR0 | XSS. Guest can allocate
>> sufficient xsave buffer based on the info.
> Please wrap changelogs closer to ~75 chars. I'm pretty sure this isn't the first
> time I've made this request...
Thanks, will keep the changelog to 70~75  chars.
>> Note, KVM does not yet support any XSS based features, i.e.
>> supported_xss is guaranteed to be zero at this time.
>>
>> Co-developed-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
>> Signed-off-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> ---
>> arch/x86/include/asm/kvm_host.h | 1 +
>> arch/x86/kvm/cpuid.c | 20 ++++++++++++++++++--
>> arch/x86/kvm/x86.c | 8 +++++---
>> 3 files changed, 24 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 28bd38303d70..20bbcd95511f 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -804,6 +804,7 @@ struct kvm_vcpu_arch {
>>
>> u64 xcr0;
>> u64 guest_supported_xcr0;
>> + u64 guest_supported_xss;
>>
>> struct kvm_pio_request pio;
>> void *pio_data;
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 7f4d13383cf2..0338316b827c 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -249,6 +249,17 @@ 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 u64 cpuid_get_supported_xss(struct kvm_cpuid_entry2 *entries, int nent)
>> +{
>> + struct kvm_cpuid_entry2 *best;
>> +
>> + best = cpuid_entry2_find(entries, nent, 0xd, 1);
>> + if (!best)
>> + return 0;
>> +
>> + return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
>> +}
>> +
>> static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,
>> int nent)
>> {
>> @@ -276,8 +287,11 @@ 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);
>> + cpuid_entry_has(best, X86_FEATURE_XSAVEC))) {
>> + u64 xstate = vcpu->arch.xcr0 | vcpu->arch.ia32_xss;
> Nit, the variable should be xfeatures, not xstate. Though I vote to avoid the
> variable entirely,
>
> 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 |
> vcpu->arch.ia32_xss, true);
>
> though it's only a slight preference, i.e. feel free to keep your approach if
> you or others feel strongly about the style.
Yes, the variable is not necessary, will remove it.
>> + }
>>
>> best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
>> if (kvm_hlt_in_guest(vcpu->kvm) && best &&
>> @@ -325,6 +339,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>>
>> vcpu->arch.guest_supported_xcr0 =
>> cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
>> + vcpu->arch.guest_supported_xss =
>> + cpuid_get_supported_xss(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
> Blech. I tried to clean up this ugly, but Paolo disagreed[*]. Can you fold in
> the below (compile tested only) patch at the very beginning of this series? It
> implements my suggested alternative. And then this would become:
>
> 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;
> }
>
> [*] https://lore.kernel.org/all/ZGfius5UkckpUyXl@google.com
Sure, will take it into my series, thanks!
>> /*
>> * FP+SSE can always be saved/restored via KVM_{G,S}ET_XSAVE, even if
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 0b9033551d8c..5d6d6fa33e5b 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3780,10 +3780,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> * 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;
>> - vcpu->arch.ia32_xss = data;
>> - kvm_update_cpuid_runtime(vcpu);
>> + if (vcpu->arch.ia32_xss != data) {
>> + vcpu->arch.ia32_xss = data;
>> + kvm_update_cpuid_runtime(vcpu);
>> + }
> Nit, I prefer this style:
>
> if (vcpu->arch.ia32_xss == data)
> break;
>
> vcpu->arch.ia32_xss = data;
> kvm_update_cpuid_runtime(vcpu);
>
> so that the common path isn't buried in an if-statement.
Yeah, I feel I'm a bit awkward to make code look nicer :-)
>> break;
>> case MSR_SMI_COUNT:
>> if (!msr_info->host_initiated)
>> --
>
> From: Sean Christopherson <seanjc@google.com>
> Date: Fri, 4 Aug 2023 08:48:03 -0700
> Subject: [PATCH] KVM: x86: Rework cpuid_get_supported_xcr0() to operate on
> vCPU data
>
> Rework and rename cpuid_get_supported_xcr0() to explicitly operate on vCPU
> state, i.e. on a vCPU's CPUID state. Prior to commit 275a87244ec8 ("KVM:
> x86: Don't adjust guest's CPUID.0x12.1 (allowed SGX enclave XFRM)"), KVM
> incorrectly fudged guest CPUID at runtime, which in turn necessitated
> massaging the incoming CPUID state for KVM_SET_CPUID{2} so as not to run
> afoul of kvm_cpuid_check_equal().
>
> Opportunistically move the helper below kvm_update_cpuid_runtime() to make
> it harder to repeat the mistake of querying supported XCR0 for runtime
> updates.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/cpuid.c | 33 ++++++++++++++++-----------------
> 1 file changed, 16 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 7f4d13383cf2..5e42846c948a 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -234,21 +234,6 @@ void kvm_update_pv_runtime(struct kvm_vcpu *vcpu)
> vcpu->arch.pv_cpuid.features = best->eax;
> }
>
> -/*
> - * Calculate guest's supported XCR0 taking into account guest CPUID data and
> - * KVM's supported XCR0 (comprised of host's XCR0 and KVM_SUPPORTED_XCR0).
> - */
> -static u64 cpuid_get_supported_xcr0(struct kvm_cpuid_entry2 *entries, int nent)
> -{
> - struct kvm_cpuid_entry2 *best;
> -
> - best = cpuid_entry2_find(entries, nent, 0xd, 0);
> - if (!best)
> - return 0;
> -
> - return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
> -}
> -
> static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,
> int nent)
> {
> @@ -299,6 +284,21 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
> }
> EXPORT_SYMBOL_GPL(kvm_update_cpuid_runtime);
>
> +/*
> + * Calculate guest's supported XCR0 taking into account guest CPUID data and
> + * KVM's supported XCR0 (comprised of host's XCR0 and KVM_SUPPORTED_XCR0).
> + */
> +static u64 vcpu_get_supported_xcr0(struct kvm_vcpu *vcpu)
> +{
> + struct kvm_cpuid_entry2 *best;
> +
> + best = kvm_find_cpuid_entry_index(vcpu, 0xd, 0);
> + if (!best)
> + return 0;
> +
> + return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
> +}
> +
> static bool kvm_cpuid_has_hyperv(struct kvm_cpuid_entry2 *entries, int nent)
> {
> struct kvm_cpuid_entry2 *entry;
> @@ -323,8 +323,7 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> kvm_apic_set_version(vcpu);
> }
>
> - vcpu->arch.guest_supported_xcr0 =
> - cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
> + vcpu->arch.guest_supported_xcr0 = vcpu_get_supported_xcr0(vcpu);
>
> /*
> * FP+SSE can always be saved/restored via KVM_{G,S}ET_XSAVE, even if
>
> base-commit: f0147fcfab840fe9a3f03e9645d25c1326373fe6