Re: [PATCH v2 12/15] KVM: x86/vmx: Disable Arch LBREn bit in #DB and warm reset

[Yang, Weijiang]

On 1/28/2023 6:09 AM, Sean Christopherson wrote:
> On Thu, Nov 24, 2022, Yang Weijiang wrote:
> > Per SDM 3B, Chapter 18:
> > “On a debug breakpoint event (#DB), IA32_LBR_CTL.LBREn is cleared.”
> > and "On a warm reset, all LBR MSRs, including IA32_LBR_DEPTH, have their
> > values preserved. However, IA32_LBR_CTL.LBREn is cleared to 0, disabling
> > LBRs.", clear the bit manually before inject #DB or when vCPU is in warm
> > reset.
> >
> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > ---
> >   arch/x86/kvm/vmx/vmx.c | 24 +++++++++++++++++++++++-
> >   1 file changed, 23 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 3bc892e8cf7a..6ad765ea4059 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -1695,6 +1695,20 @@ static void vmx_clear_hlt(struct kvm_vcpu *vcpu)
> >   		vmcs_write32(GUEST_ACTIVITY_STATE, GUEST_ACTIVITY_ACTIVE);
> >   }
> >   
> > +static void disable_arch_lbr_ctl(struct kvm_vcpu *vcpu)
> > +{
> > +	struct lbr_desc *lbr_desc = vcpu_to_lbr_desc(vcpu);
> > +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> > +
> > +	if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR) &&
> > +	    test_bit(INTEL_PMC_IDX_FIXED_VLBR, pmu->pmc_in_use) &&
> > +	    lbr_desc->event) {
> I don't see any reason to check that an event is actually assigned.  The behavior
> is architectural, whether or not KVM is actively exposing LBRs to the guest is
> irrelevant

Agree, will remove them.

> > +		u64 ctl = vmcs_read64(GUEST_IA32_LBR_CTL);
> > +
> > +		vmcs_write64(GUEST_IA32_LBR_CTL, ctl & ~ARCH_LBR_CTL_LBREN);
> > +	}
> > +}
> > +
> >   static void vmx_inject_exception(struct kvm_vcpu *vcpu)
> >   {
> >   	struct kvm_queued_exception *ex = &vcpu->arch.exception;
> > @@ -1738,6 +1752,9 @@ static void vmx_inject_exception(struct kvm_vcpu *vcpu)
> >   	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, intr_info);
> >   
> >   	vmx_clear_hlt(vcpu);
> > +
> > +	if (ex->vector == DB_VECTOR)
> > +		disable_arch_lbr_ctl(vcpu);
> >   }
> >   
> >   static void vmx_setup_uret_msr(struct vcpu_vmx *vmx, unsigned int msr,
> > @@ -4796,7 +4813,9 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> >   
> >   	vmx_update_fb_clear_dis(vcpu, vmx);
> >   
> > -	if (!init_event && cpu_has_vmx_arch_lbr())
> > +	if (init_event)
> INIT and warm RESET are not the same thing, i.e. this is flat out wrong.

I was confused a bit. Is there's a way to intercept guest warm RESET so 
as to disable the bit?

Or what should be followed per spec.?

> > +		disable_arch_lbr_ctl(vcpu);
> > +	else if (cpu_has_vmx_arch_lbr())
> >   		vmcs_write64(GUEST_IA32_LBR_CTL, 0);
> >   }
> >   
> > @@ -4873,6 +4892,9 @@ static void vmx_inject_nmi(struct kvm_vcpu *vcpu)
> >   			INTR_TYPE_NMI_INTR | INTR_INFO_VALID_MASK | NMI_VECTOR);
> >   
> >   	vmx_clear_hlt(vcpu);
> > +
> > +	if (vcpu->arch.exception.vector == DB_VECTOR)
> Huh?  This is _very_ obviously injecting NMIs, not #DBs.

My fault, will remove it.

> > +		disable_arch_lbr_ctl(vcpu);
> >   }
> >   
> >   bool vmx_get_nmi_mask(struct kvm_vcpu *vcpu)
> > --
> > 2.27.0