Re: [PATCH 3/5] KVM: VMX: RTIT_CTL_BRANCH_EN has no dependency on other CPUID bit

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 8/25/2021 11:30 AM, Like Xu wrote:
> +Alexander
> 
> On 24/8/2021 7:07 pm, Xiaoyao Li wrote:
>> Per Intel SDM, RTIT_CTL_BRANCH_EN bit has no dependency on any CPUID
>> leaf 0x14.
>>
>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
>> ---
>>   arch/x86/kvm/vmx/vmx.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 7ed96c460661..4a70a6d2f442 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -7116,7 +7116,8 @@ static void update_intel_pt_cfg(struct kvm_vcpu 
>> *vcpu)
>>       /* Initialize and clear the no dependency bits */
>>       vmx->pt_desc.ctl_bitmask = ~(RTIT_CTL_TRACEEN | RTIT_CTL_OS |
>> -            RTIT_CTL_USR | RTIT_CTL_TSC_EN | RTIT_CTL_DISRETC);
>> +            RTIT_CTL_USR | RTIT_CTL_TSC_EN | RTIT_CTL_DISRETC |
>> +            RTIT_CTL_BRANCH_EN);
>>       /*
>>        * If CPUID.(EAX=14H,ECX=0):EBX[0]=1 CR3Filter can be set otherwise
>> @@ -7134,12 +7135,11 @@ static void update_intel_pt_cfg(struct 
>> kvm_vcpu *vcpu)
>>                   RTIT_CTL_CYC_THRESH | RTIT_CTL_PSB_FREQ);
>>       /*
>> -     * If CPUID.(EAX=14H,ECX=0):EBX[3]=1 MTCEn BranchEn and
>> -     * MTCFreq can be set
>> +     * If CPUID.(EAX=14H,ECX=0):EBX[3]=1 MTCEn and MTCFreq can be set
> 
> If CPUID.(EAX=14H,ECX=0):EBX[3]=1,
> 
>      "indicates support of MTC timing packet and suppression of 
> COFI-based packets."

I think it's a mistake of SDM in CPUID instruction.

If you read 31.3.1, table 31-11 of SDM 325462-075US,

It just says CPUID(0x14, 0):EBX[3]: MTC supprted.
It doesn't talk anything about COFI packets suppression.

Further as below.

> Per 31.2.5.4 Branch Enable (BranchEn),
> 
>      "If BranchEn is not set, then relevant COFI packets (TNT, TIP*, 
> FUP, MODE.*) are suppressed."
> 
> I think if the COFI capability is suppressed, the software can't set the 
> BranchEn bit, right ?

Based on your understanding, isn't it that

1. if CPUID.(EAX=14H,ECX=0):EBX[3]=0, it doesn't support "suppression of 
COFI-based packets".
2. if it doesn't support "suppression of COFI-based packets", then it 
doens't support "If BranchEn is not set, then relevant COFI packets 
(TNT, TIP*, FUP, MODE.*) are suppressed", i.e. BranchEn must be 1.

Anyway, I think it's just a mistake on CPUID instruction document of SDM.

CPUD.(EAX=14H,ECX=0):EBX[3] should only indicates the MTC support.

BranchEn should be always supported if PT is available. Per "31.2.7.2 
IA32_RTIT_CTL MSR" on SDM:
When BranchEn is 1, it enables COFI-based packets.
When BranchEn is 0, it disables COFI-based packtes. i.e., COFI packets 
are suppressed.

>>        */
>>       if (intel_pt_validate_cap(vmx->pt_desc.caps, PT_CAP_mtc))
>>           vmx->pt_desc.ctl_bitmask &= ~(RTIT_CTL_MTC_EN |
>> -                RTIT_CTL_BRANCH_EN | RTIT_CTL_MTC_RANGE);
>> +                          RTIT_CTL_MTC_RANGE);
>>       /* If CPUID.(EAX=14H,ECX=0):EBX[4]=1 FUPonPTW and PTWEn can be 
>> set */
>>       if (intel_pt_validate_cap(vmx->pt_desc.caps, PT_CAP_ptwrite))
>>