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

[Like Xu <like.xu.linux@gmail.com>]

On 25/8/2021 12:19 pm, Xiaoyao Li wrote:
> 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.

That's it.

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

Is this an ambiguity rather than a mistake ?

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

Please do not make assertions that you do not confirm with hw.

> 
> BranchEn should be always supported if PT is available. Per "31.2.7.2 

Check d35869ba348d3f1ff3e6d8214fe0f674bb0e404e.

> 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))
>>>
>