linux-kernel - Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

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Date: Fri, 17 May 2024 16:04:38 +0800
From: "Yang, Weijiang" <weijiang.yang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: <pbonzini@...hat.com>, <dave.hansen@...el.com>, <x86@...nel.org>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<peterz@...radead.org>, <chao.gao@...el.com>, <rick.p.edgecombe@...el.com>,
	<mlevitsk@...hat.com>, <john.allen@....com>
Subject: Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and
 advertise to userspace

On 5/16/2024 10:43 PM, Sean Christopherson wrote:
> On Thu, May 16, 2024, Weijiang Yang wrote:
>> On 5/6/2024 5:41 PM, Yang, Weijiang wrote:
>>> On 5/2/2024 7:34 AM, Sean Christopherson wrote:
>>>> On Sun, Feb 18, 2024, Yang Weijiang wrote:
>>>>> @@ -665,7 +665,7 @@ void kvm_set_cpu_caps(void)
>>>>>            F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
>>>>>            F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
>>>>>            F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | 0 /*WAITPKG*/ |
>>>>> -        F(SGX_LC) | F(BUS_LOCK_DETECT)
>>>>> +        F(SGX_LC) | F(BUS_LOCK_DETECT) | F(SHSTK)
>>>>>        );
>>>>>        /* Set LA57 based on hardware capability. */
>>>>>        if (cpuid_ecx(7) & F(LA57))
>>>>> @@ -683,7 +683,8 @@ void kvm_set_cpu_caps(void)
>>>>>            F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES) | F(INTEL_STIBP) |
>>>>>            F(MD_CLEAR) | F(AVX512_VP2INTERSECT) | F(FSRM) |
>>>>>            F(SERIALIZE) | F(TSXLDTRK) | F(AVX512_FP16) |
>>>>> -        F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16) | F(FLUSH_L1D)
>>>>> +        F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16) | F(FLUSH_L1D) |
>>>>> +        F(IBT)
>>>>>        );
>>>> ...
>>>>
>>>>> @@ -7977,6 +7993,18 @@ static __init void vmx_set_cpu_caps(void)
>>>>>          if (cpu_has_vmx_waitpkg())
>>>>>            kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG);
>>>>> +
>>>>> +    /*
>>>>> +     * Disable CET if unrestricted_guest is unsupported as KVM doesn't
>>>>> +     * enforce CET HW behaviors in emulator. On platforms with
>>>>> +     * VMX_BASIC[bit56] == 0, inject #CP at VMX entry with error code
>>>>> +     * fails, so disable CET in this case too.
>>>>> +     */
>>>>> +    if (!cpu_has_load_cet_ctrl() || !enable_unrestricted_guest ||
>>>>> +        !cpu_has_vmx_basic_no_hw_errcode()) {
>>>>> +        kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
>>>>> +        kvm_cpu_cap_clear(X86_FEATURE_IBT);
>>>>> +    }
>>>> Oh!  Almost missed it.  This patch should explicitly kvm_cpu_cap_clear()
>>>> X86_FEATURE_SHSTK and X86_FEATURE_IBT.  We *know* there are upcoming AMD CPUs
>>>> that support at least SHSTK, so enumerating support for common code would yield
>>>> a version of KVM that incorrectly advertises support for SHSTK.
>>>>
>>>> I hope to land both Intel and AMD virtualization in the same kernel release, but
>>>> there are no guarantees that will happen.  And explicitly clearing both SHSTK and
>>>> IBT would guard against IBT showing up in some future AMD CPU in advance of KVM
>>>> gaining full support.
>>> Let me be clear on this, you want me to disable SHSTK/IBT with
>>> kvm_cpu_cap_clear() unconditionally for now in this patch, and wait until
>>> both AMD's SVM patches and this series are ready for guest CET, then remove
>>> the disabling code in this patch for final merge, am I right?
> No, allow it to be enabled for VMX, but explicitly disable it for SVM, i.e.
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 4aaffbf22531..b3df12af4ee6 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5125,6 +5125,10 @@ static __init void svm_set_cpu_caps(void)
>          kvm_caps.supported_perf_cap = 0;
>          kvm_caps.supported_xss = 0;
>   
> +       /* KVM doesn't yet support CET virtualization for SVM. */
> +       kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
> +       kvm_cpu_cap_clear(X86_FEATURE_IBT);
> +
>          /* CPUID 0x80000001 and 0x8000000A (SVM features) */
>          if (nested) {
>                  kvm_cpu_cap_set(X86_FEATURE_SVM);
>
> Then the SVM series can simply delete those lines when all is ready.

Understood, thanks!