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

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Message-ID: <39b95ac6-f163-4461-93f3-eaa653ab1355@intel.com>
Date: Thu, 16 May 2024 15:13:01 +0800
From: "Yang, Weijiang" <weijiang.yang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, <rick.p.edgecombe@...el.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>, <mlevitsk@...hat.com>,
	<john.allen@....com>
Subject: Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and
 advertise to userspace

On 5/2/2024 7:15 AM, Sean Christopherson wrote:
> On Sun, Feb 18, 2024, Yang Weijiang wrote:
>> @@ -696,6 +697,20 @@ void kvm_set_cpu_caps(void)
>>   		kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP);
>>   	if (boot_cpu_has(X86_FEATURE_AMD_SSBD))
>>   		kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
>> +	/*
>> +	 * Don't use boot_cpu_has() to check availability of IBT because the
>> +	 * feature bit is cleared in boot_cpu_data when ibt=off is applied
>> +	 * in host cmdline.
> I'm not convinced this is a good reason to diverge from the host kernel.  E.g.
> PCID and many other features honor the host setup, I don't see what makes IBT
> special.
>
>
Hi, Sean,
We synced the issue internally, and got conclusion that KVM should honor host IBT config.
In this case IBT bit in boot_cpu_data should be honored.  With this policy, it can avoid CPUID
confusion to guest side due to host ibt=off config. Host side xstate support couldn't be an issue
because we already have below check in this patch:

+ if ((kvm_caps.supported_xss & (XFEATURE_MASK_CET_USER | + XFEATURE_MASK_CET_KERNEL)) != + (XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL)) { + kvm_cpu_cap_clear(X86_FEATURE_SHSTK); + kvm_cpu_cap_clear(X86_FEATURE_IBT); + kvm_caps.supported_xss &= ~(XFEATURE_MASK_CET_USER | + XFEATURE_MASK_CET_KERNEL); + }

What's your thoughts? Should I just remove the quirk here and keep everything normal and
peaceful?