Re: [RFC PATCH 2/3] kvm: x86: Use KVM_DEBUGREG_NEED_RELOAD instead of KVM_DEBUGREG_BP_ENABLED

[Xiaoyao Li]

On 4/24/2020 3:29 AM, Sean Christopherson wrote:
> On Thu, Apr 16, 2020 at 06:15:08PM +0800, Xiaoyao Li wrote:
> > Once any #BP enabled in DR7, it will set KVM_DEBUGREG_BP_ENABLED, which
> > leads to reload DRn before every VM entry even if none of DRn changed.
> >
> > Drop KVM_DEBUGREG_BP_ENABLED flag and set KVM_DEBUGREG_NEED_RELOAD flag
> > for the cases that DRn need to be reloaded instead, to avoid unnecessary
> > DRn reload.
>
> Loading DRs on every VM-Enter _is_ necessary if there are breakpoints
> enabled for the guest.  The hardware DR values are not "stable", e.g. they
> are loaded with the host's values immediately after saving the guest's
> value (if DR_EXITING is disabled) in vcpu_enter_guest(), notably iff the
> host has an active/enabled breakpoint.  

May bad, bbviously I didn't think about it.

> My bet is that DRs can be changed
> from interrupt context as well.
So set KVM_DEBUGREG_NEED_RELOAD in vcpu_load won't help.

> Loading DRs for the guest (not necessarily the same as the guest's DRs) is
> necessary if a breakpoint is enabled so that the #DB is actually hit in
> guest.  It's a similar concept to instructions that consume MSR values,
> e.g. SYSCALL, RDTSCP, etc..., even if KVM intercepts the MSR/DR, hardware
> still needs the correct value so that the guest behavior is correct.
>
> > Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > ---
> >   arch/x86/include/asm/kvm_host.h | 3 +--
> >   arch/x86/kvm/x86.c              | 4 ++--
> >   2 files changed, 3 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index f465c76e6e5a..87e2d020351e 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -509,9 +509,8 @@ struct kvm_pmu {
> >   struct kvm_pmu_ops;
> >   
> >   enum {
> > -	KVM_DEBUGREG_BP_ENABLED = 1,
> > +	KVM_DEBUGREG_NEED_RELOAD = 1,
> >   	KVM_DEBUGREG_WONT_EXIT = 2,
> > -	KVM_DEBUGREG_NEED_RELOAD = 4,
> >   };
> >   
> >   struct kvm_mtrr_range {
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index cce926658d10..71264df64001 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1086,9 +1086,8 @@ static void kvm_update_dr7(struct kvm_vcpu *vcpu)
> >   	else
> >   		dr7 = vcpu->arch.dr7;
> >   	kvm_x86_ops.set_dr7(vcpu, dr7);
> > -	vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_BP_ENABLED;
> >   	if (dr7 & DR7_BP_EN_MASK)
> > -		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_BP_ENABLED;
> > +		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_NEED_RELOAD;
> >   }
> >   
> >   static u64 kvm_dr6_fixed(struct kvm_vcpu *vcpu)
> > @@ -1128,6 +1127,7 @@ static int __kvm_set_dr(struct kvm_vcpu *vcpu, int dr, unsigned long val)
> >   		break;
> >   	}
> >   
> > +	vcpu->arch.switch_db_regs |= KVM_DEBUGREG_NEED_RELOAD;
> >   	return 0;
> >   }
> >   
> > --
> > 2.20.1