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

[Xiaoyao Li]

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@xxxxxxxxx>
> > > > ---
> > > >   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))