From: Xiaoyao Li <xiaoyao.li@intel.com>
To: Like Xu <like.xu.linux@gmail.com>
Cc: Sean Christopherson <seanjc@google.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Paolo Bonzini <pbonzini@redhat.com>,
"Alexander Shishkin (hwtracing + intel_th + stm + R:perf)"
<alexander.shishkin@linux.intel.com>
Subject: Re: [PATCH 3/5] KVM: VMX: RTIT_CTL_BRANCH_EN has no dependency on other CPUID bit
Date: Wed, 25 Aug 2021 16:58:09 +0800 [thread overview]
Message-ID: <d3ee1f77-8e15-f24f-d55f-cea272faf55c@intel.com> (raw)
In-Reply-To: <26103eda-806a-516d-096f-c9e85552286a@gmail.com>
On 8/25/2021 4:14 PM, Like Xu wrote:
> 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@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.
>
> 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.
Isn't this patch fixing real problem? Without it, it forbids guest to
enable BranchEn if PT_MTC_cap not exposed to guest.
next prev parent reply other threads:[~2021-08-25 8:58 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-24 11:07 [PATCH 0/5] KVM: VMX: PT (processor trace) optimizations and fixes Xiaoyao Li
2021-08-24 11:07 ` [PATCH 1/5] KVM: VMX: Restore host's MSR_IA32_RTIT_CTL when it's not zero Xiaoyao Li
2021-08-24 17:54 ` Sean Christopherson
2021-08-24 11:07 ` [PATCH 2/5] KVM: VMX: Use cached vmx->pt_desc.addr_range Xiaoyao Li
2021-08-24 15:24 ` Sean Christopherson
2021-08-24 15:42 ` Xiaoyao Li
2021-08-24 11:07 ` [PATCH 3/5] KVM: VMX: RTIT_CTL_BRANCH_EN has no dependency on other CPUID bit Xiaoyao Li
2021-08-25 3:30 ` Like Xu
2021-08-25 4:19 ` Xiaoyao Li
2021-08-25 6:08 ` Like Xu
2021-08-25 6:33 ` Xiaoyao Li
2021-08-25 8:14 ` Like Xu
2021-08-25 8:58 ` Xiaoyao Li [this message]
2021-08-25 11:53 ` Alexander Shishkin
2021-08-24 11:07 ` [PATCH 4/5] KVM: VMX: Disallow PT MSRs accessing if PT is not exposed to guest Xiaoyao Li
2021-08-24 14:20 ` Sean Christopherson
2021-08-24 15:35 ` Xiaoyao Li
2021-08-24 16:48 ` Sean Christopherson
2021-08-24 11:07 ` [PATCH 5/5] KVM: VMX: Check Intel PT related CPUID leaves Xiaoyao Li
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