Re: [PATCH 2/2] KVM: vmx/pmu: Clear DEBUGCTLMSR_LBR bit on the debug breakpoint event

[Paolo Bonzini]

On 23/02/21 17:38, Sean Christopherson wrote:
> On Tue, Feb 23, 2021, Like Xu wrote:
> > When the processor that support model-specific LBR generates a debug
> > breakpoint event, it automatically clears the LBR flag. This action
> > does not clear previously stored LBR stack MSRs. (Intel SDM 17.4.2)
> >
> > Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/vmx/vmx.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index e0a3a9be654b..4951b535eb7f 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -4795,6 +4795,7 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
> >   	u32 intr_info, ex_no, error_code;
> >   	unsigned long cr2, rip, dr6;
> >   	u32 vect_info;
> > +	u64 lbr_ctl;
> >   
> >   	vect_info = vmx->idt_vectoring_info;
> >   	intr_info = vmx_get_intr_info(vcpu);
> > @@ -4886,6 +4887,10 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
> >   		rip = kvm_rip_read(vcpu);
> >   		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
> >   		kvm_run->debug.arch.exception = ex_no;
> > +		/* On the debug breakpoint event, the LBREn bit is cleared. */
>
> Except this code is in BP_VECTOR, not DB_VECTOR as it should be.
>
>    When the processor generates a debug exception (#DB), it automatically clears
>    the LBR flag before executing the exception handler. This action does not
>    clear previously stored LBR stack MSRs.

Also, this should come with a testcase.

Paolo