On Tue, Sep 01, 2009 at 01:28:46PM +0200, Jan Kiszka wrote:
> Marcelo Tosatti wrote:
> > On Tue, Sep 01, 2009 at 12:44:08PM +0300, Avi Kivity wrote:
> >> Instead of saving the debug registers from the processor to a kvm data
> >> structure, rely in the debug registers stored in the thread structure.
> >> This allows us not to save dr6 and dr7.
> >>
> >> Reduces lightweight vmexit cost by 350 cycles, or 11 percent.
> >
> > Is this kgdb safe?
>
> Nope, kgdb writes directly to the debug registers.
>
> I vaguely recall someone trying to push a debug register management
> framework. Did it hit mainline in the meantime? I do not find any trace
> on quick glance, at least not in kgdb.
A simple kgdb_enabled sort of flag, in addition to TIF_DEBUG would do
it?
> Jan
>
> >
> >> Signed-off-by: Avi Kivity <avi@xxxxxxxxxx>
> >> ---
> >> arch/x86/include/asm/kvm_host.h | 3 ---
> >> arch/x86/kvm/x86.c | 22 +++++++---------------
> >> 2 files changed, 7 insertions(+), 18 deletions(-)
> >>
> >> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> >> index 6046e6f..45226f0 100644
> >> --- a/arch/x86/include/asm/kvm_host.h
> >> +++ b/arch/x86/include/asm/kvm_host.h
> >> @@ -362,9 +362,6 @@ struct kvm_vcpu_arch {
> >> u32 pat;
> >>
> >> int switch_db_regs;
> >> - unsigned long host_db[KVM_NR_DB_REGS];
> >> - unsigned long host_dr6;
> >> - unsigned long host_dr7;
> >> unsigned long db[KVM_NR_DB_REGS];
> >> unsigned long dr6;
> >> unsigned long dr7;
> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >> index 891234b..9e3acbd 100644
> >> --- a/arch/x86/kvm/x86.c
> >> +++ b/arch/x86/kvm/x86.c
> >> @@ -3627,14 +3627,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >>
> >> kvm_guest_enter();
> >>
> >> - get_debugreg(vcpu->arch.host_dr6, 6);
> >> - get_debugreg(vcpu->arch.host_dr7, 7);
> >> if (unlikely(vcpu->arch.switch_db_regs)) {
> >> - get_debugreg(vcpu->arch.host_db[0], 0);
> >> - get_debugreg(vcpu->arch.host_db[1], 1);
> >> - get_debugreg(vcpu->arch.host_db[2], 2);
> >> - get_debugreg(vcpu->arch.host_db[3], 3);
> >> -
> >> set_debugreg(0, 7);
> >> set_debugreg(vcpu->arch.eff_db[0], 0);
> >> set_debugreg(vcpu->arch.eff_db[1], 1);
> >> @@ -3645,15 +3638,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >> trace_kvm_entry(vcpu->vcpu_id);
> >> kvm_x86_ops->run(vcpu);
> >>
> >> - if (unlikely(vcpu->arch.switch_db_regs)) {
> >> - set_debugreg(0, 7);
> >> - set_debugreg(vcpu->arch.host_db[0], 0);
> >> - set_debugreg(vcpu->arch.host_db[1], 1);
> >> - set_debugreg(vcpu->arch.host_db[2], 2);
> >> - set_debugreg(vcpu->arch.host_db[3], 3);
> >> + if (unlikely(vcpu->arch.switch_db_regs || test_thread_flag(TIF_DEBUG))) {
> >> + set_debugreg(current->thread.debugreg0, 0);
> >> + set_debugreg(current->thread.debugreg1, 1);
> >> + set_debugreg(current->thread.debugreg2, 2);
> >> + set_debugreg(current->thread.debugreg3, 3);
> >> + set_debugreg(current->thread.debugreg6, 6);
> >> + set_debugreg(current->thread.debugreg7, 7);
> >> }
> >> - set_debugreg(vcpu->arch.host_dr6, 6);
> >> - set_debugreg(vcpu->arch.host_dr7, 7);
> >>
> >> set_bit(KVM_REQ_KICK, &vcpu->requests);
> >> local_irq_enable();
> >> --
> >> 1.6.4.1
>
> --
> Siemens AG, Corporate Technology, CT SE 2
> Corporate Competence Center Embedded Linux
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