linux-kernel - Re: [RFC PATCH v2 06/11] KVM: x86: Advertise ARCH_CAP_VIRTUAL

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Date:   Mon, 22 May 2023 09:02:00 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Chao Gao <chao.gao@...el.com>
Cc:     kvm@...r.kernel.org, Jiaan Lu <jiaan.lu@...el.com>,
        Zhang Chen <chen.zhang@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 06/11] KVM: x86: Advertise ARCH_CAP_VIRTUAL_ENUM
 support

On 5/19/2023 5:57 PM, Chao Gao wrote:
> On Thu, May 18, 2023 at 06:14:40PM +0800, Xiaoyao Li wrote:
>>>    static u32 msr_based_features[ARRAY_SIZE(msr_based_features_all)];
>>> @@ -1591,7 +1593,8 @@ static unsigned int num_msr_based_features;
>>>    	 ARCH_CAP_SKIP_VMENTRY_L1DFLUSH | ARCH_CAP_SSB_NO | ARCH_CAP_MDS_NO | \
>>>    	 ARCH_CAP_PSCHANGE_MC_NO | ARCH_CAP_TSX_CTRL_MSR | ARCH_CAP_TAA_NO | \
>>>    	 ARCH_CAP_SBDR_SSDP_NO | ARCH_CAP_FBSDP_NO | ARCH_CAP_PSDP_NO | \
>>> -	 ARCH_CAP_FB_CLEAR | ARCH_CAP_RRSBA | ARCH_CAP_PBRSB_NO)
>>> +	 ARCH_CAP_FB_CLEAR | ARCH_CAP_RRSBA | ARCH_CAP_PBRSB_NO | \
>>> +	 ARCH_CAP_VIRTUAL_ENUM)
>>
>> We cannot do it.
>>
>> Otherwise, an AMD L1 with X86_FEATURE_ARCH_CAPABILITIES configured is
>> possible to expose MSR_VIRTUAL_ENUMERATION to L2 while no support for it.
> 
> How does AMD L1 see the ARCH_CAP_VIRTUAL_ENUM feature in the first
> place? because ...
> 
>>
>>>    static u64 kvm_get_arch_capabilities(void)
>>>    {
>>> @@ -1610,6 +1613,17 @@ static u64 kvm_get_arch_capabilities(void)
>>>    	 */
>>>    	data |= ARCH_CAP_PSCHANGE_MC_NO;
>>> +	/*
>>> +	 * Virtual enumeration is a paravirt feature. The only usage for now
>>> +	 * is to bridge the gap caused by microarchitecture changes between
>>> +	 * different Intel processors. And its usage is linked to "virtualize
>>> +	 * IA32_SPEC_CTRL" which is a VMX feature. Whether AMD SVM can benefit
>>> +	 * from the same usage and how to implement it is still unclear. Limit
>>> +	 * virtual enumeration to VMX.
>>> +	 */
>>> +	if (static_call(kvm_x86_has_emulated_msr)(NULL, MSR_VIRTUAL_ENUMERATION))
>>> +		data |= ARCH_CAP_VIRTUAL_ENUM;
> 
> the feature is exposed on Intel CPUs only.
> 
> Do you mean AMD L1 created on Intel L0? and Intel L0 even emulates
> nested (SVM) support for the L1? This sounds a very contrived case.

you are right. I was thinking of an rare case but ignored the fact that 
VMX doesn't nested svm.

Sorry for it.

>>> +
>>>    	/*
>>>    	 * If we're doing cache flushes (either "always" or "cond")
>>>    	 * we will do one whenever the guest does a vmlaunch/vmresume.
>>