Re: [PATCH v5 08/19] KVM:x86: Report KVM supported CET MSRs as to-be-saved

Yang, Weijiang
2023-08-06T08:55:21Z

On 8/4/2023 1:51 PM, Chao Gao wrote:
> On Fri, Aug 04, 2023 at 11:13:36AM +0800, Yang, Weijiang wrote:
>>>> @@ -7214,6 +7217,13 @@ static void kvm_probe_msr_to_save(u32 msr_index)
>>>> if (!kvm_caps.supported_xss)
>>>> return;
>>>> break;
>>>> + case MSR_IA32_U_CET:
>>>> + case MSR_IA32_S_CET:
>>>> + case MSR_KVM_GUEST_SSP:
>>>> + case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
>>>> + if (!kvm_is_cet_supported())
>>> shall we consider the case where IBT is supported while SS isn't
>>> (e.g., in L1 guest)?
>> Yes, but userspace should be able to access SHSTK MSRs even only IBT is exposed to guest so
>> far as KVM can support SHSTK MSRs.
> Why should userspace be allowed to access SHSTK MSRs in this case? L1 may not
> even enumerate SHSTK (qemu removes -shstk explicitly but keeps IBT), how KVM in
> L1 can allow its userspace to do that?
Hold on until host_initiated access is finalized.
>>>> +static inline bool kvm_is_cet_supported(void)
>>>> +{
>>>> + return (kvm_caps.supported_xss & CET_XSTATE_MASK) == CET_XSTATE_MASK;
>>> why not just check if SHSTK or IBT is supported explicitly, i.e.,
>>>
>>> return kvm_cpu_cap_has(X86_FEATURE_SHSTK) ||
>>> kvm_cpu_cap_has(X86_FEATURE_IBT);
>>>
>>> this is straightforward. And strictly speaking, the support of a feature and
>>> the support of managing a feature's state via XSAVE(S) are two different things.x
>> I think using exiting check implies two things:
>> 1. Platform/KVM can support CET features.
>> 2. CET user mode MSRs are backed by host thus are guaranteed to be valid.
>> i.e., the purpose is to check guest CET dependencies instead of features' availability.
> When KVM claims a feature is supported, it should ensure all its dependencies are
> met. that's, KVM's support of a feature also imples all dependencies are met.
> Function-wise, the two approaches have no difference. I just think checking
> KVM's support of SHSTK/IBT is more clear because the function name is
> kvm_is_cet_supported() rather than e.g., kvm_is_cet_state_managed_by_xsave().
OK, maybe the helper is not necessary anymore, I will remove it, thank you!
>> kvm_cpu_cap_has(X86_FEATURE_SHSTK) || kvm_cpu_cap_has(X86_FEATURE_IBT)
>>
>> only tells at least one of the CET features is supported by KVM.
>>
>>> then patch 16 has no need to do
>>>
>>> + /*
>>> + * If SHSTK and IBT are not available in KVM, clear CET user bit in
>>> + * kvm_caps.supported_xss so that kvm_is_cet__supported() returns
>>> + * false when called.
>>> + */
>>> + if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
>>> + !kvm_cpu_cap_has(X86_FEATURE_IBT))
>>> + kvm_caps.supported_xss &= ~CET_XSTATE_MASK;