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

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Message-ID: <26103eda-806a-516d-096f-c9e85552286a@gmail.com>
Date:   Wed, 25 Aug 2021 16:14:02 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        "Alexander Shishkin (hwtracing + intel_th + stm + R:perf)" 
        <alexander.shishkin@...ux.intel.com>
Subject: Re: [PATCH 3/5] KVM: VMX: RTIT_CTL_BRANCH_EN has no dependency on
 other CPUID bit

On 25/8/2021 2:33 pm, Xiaoyao Li wrote:
> 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@...el.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.

Per Vol. 4 Table 2-34. Additional MSRs Common to Processors Based the
Broadwell Microarchitectures, the BranchEn bit 13 is:

	"Reserved; writing 0 will #GP if also setting TraceEn"

on the Intel® Core™ M Processors.

My point is that we, especially software developers from hardware vendors,
should really focus on real hardware and fix real problems.

<EOM>

> 
>>> 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))
>>>>>
>>>
>