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Date:   Sat, 5 Aug 2023 00:01:15 +0200
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To:     Sean Christopherson <seanjc@google.com>,
        Chao Gao <chao.gao@intel.com>
Cc:     Weijiang Yang <weijiang.yang@intel.com>, peterz@infradead.org,
        john.allen@amd.com, kvm@vger.kernel.org,
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        binbin.wu@linux.intel.com
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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v5 08/19] KVM:x86: Report KVM supported CET MSRs as
 to-be-saved
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On 8/4/23 20:51, Sean Christopherson wrote:
>>>>> +	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?
>
> +1.  And specifically, this isn't about SHSTK being exposed to the guest, it's about
> SHSTK being _supported by KVM_.  This is all about KVM telling userspace what MSRs
> are valid and/or need to be saved+restored.  If KVM doesn't support a feature,
> then the MSRs are invalid and there is no reason for userspace to save+restore
> the MSRs on live migration.

I think you three are talking past each other.

There are four cases:

- U_CET/S_CET supported by the host and exposed (obvious).

- U_CET/S_CET supported by the host, IBT or SHSTK partially exposed.  The
MSRs should still be guest-accessible and bits that apply to absent features
should be reserved (bits 0-1 for SHSTK, bits 2-63 for IBT).

- U_CET/S_CET supported by the host, IBT or SHSTK not exposed.  The MSRs
should still be host-accessible and writable to the default value.  This is
clearer if you think that KVM_GET_MSR_INDEX_LIST is a system ioctl.  Whether
to allow writing 0 from the guest is debatable.

- U_CET/S_CET not supported by the host.  Then the MSRs should not be
enabled and should not be in KVM_GET_MSR_INDEX_LIST, and also IBT/SHSTK
should not be in KVM_GET_SUPPORTED_CPUID.

In my opinion it is reasonable to require both U_CET and S_CET to be
supported from the beginning in the host in order to support CET.  It
is simpler and keeps the feature matrix at bay.

Paolo