Messages in this thread | | | Subject | Re: [PATCH 3/5] KVM: VMX: RTIT_CTL_BRANCH_EN has no dependency on other CPUID bit | From | Xiaoyao Li <> | Date | Wed, 25 Aug 2021 12:19:25 +0800 |
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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)) >>
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