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 2:08 PM, Like Xu wrote:
> 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.

This commit shows BranchEn is supported on BDW, and must be enabled on 
BDW. This doesn't conflict the description above that BranchEn should be 
always supported.

>> 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))
>>>>
>>