| Message ID | 1251810385-21050-1-git-send-email-avi@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Avi Kivity wrote: > Only reload debug register 6 if we're running with the guest's > debug registers. Saves around 150 cycles from the guest lightweight > exit path. > > dr6 contains a couple of bits that are updated on #DB, so intercept > that unconditionally and update those bits then. > > Signed-off-by: Avi Kivity <avi@redhat.com> > --- > v2: trap #DB so we maintain the TF related bits of DR7. > > arch/x86/kvm/vmx.c | 14 +++++++++----- > 1 files changed, 9 insertions(+), 5 deletions(-) > > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > index 05cd554..d4be978 100644 > --- a/arch/x86/kvm/vmx.c > +++ b/arch/x86/kvm/vmx.c > @@ -540,10 +540,12 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu) > eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR); > if (!vcpu->fpu_active) > eb |= 1u << NM_VECTOR; > + /* > + * Unconditionally intercept #DB so we can maintain dr6 without > + * reading it every exit. > + */ > + eb |= 1u << DB_VECTOR; If this is safe... > if (vcpu->guest_debug & KVM_GUESTDBG_ENABLE) { > - if (vcpu->guest_debug & > - (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) > - eb |= 1u << DB_VECTOR; > if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP) > eb |= 1u << BP_VECTOR; > } > @@ -3629,7 +3631,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) > */ > vmcs_writel(HOST_CR0, read_cr0()); > > - set_debugreg(vcpu->arch.dr6, 6); > + if (vcpu->arch.switch_db_regs) > + set_debugreg(vcpu->arch.dr6, 6); > > asm( > /* Store host registers */ > @@ -3731,7 +3734,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) > | (1 << VCPU_EXREG_PDPTR)); > vcpu->arch.regs_dirty = 0; > > - get_debugreg(vcpu->arch.dr6, 6); > + if (vcpu->arch.switch_db_regs) > + get_debugreg(vcpu->arch.dr6, 6); > > vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD); > if (vmx->rmode.irq.pending) ...I wonder why we cannot drop this save/restore? The guest is not able to access dr6 without causing a trap, thus will never see dr6 as stored in the hardware. Jan
On 09/01/2009 04:15 PM, Jan Kiszka wrote: > Avi Kivity wrote: > >> Only reload debug register 6 if we're running with the guest's >> debug registers. Saves around 150 cycles from the guest lightweight >> exit path. >> >> dr6 contains a couple of bits that are updated on #DB, so intercept >> that unconditionally and update those bits then. >> >> Signed-off-by: Avi Kivity<avi@redhat.com> >> --- >> v2: trap #DB so we maintain the TF related bits of DR7. >> >> arch/x86/kvm/vmx.c | 14 +++++++++----- >> 1 files changed, 9 insertions(+), 5 deletions(-) >> >> >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c >> index 05cd554..d4be978 100644 >> --- a/arch/x86/kvm/vmx.c >> +++ b/arch/x86/kvm/vmx.c >> @@ -540,10 +540,12 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu) >> eb = (1u<< PF_VECTOR) | (1u<< UD_VECTOR) | (1u<< MC_VECTOR); >> if (!vcpu->fpu_active) >> eb |= 1u<< NM_VECTOR; >> + /* >> + * Unconditionally intercept #DB so we can maintain dr6 without >> + * reading it every exit. >> + */ >> + eb |= 1u<< DB_VECTOR; >> > If this is safe... > > >> if (vcpu->guest_debug& KVM_GUESTDBG_ENABLE) { >> - if (vcpu->guest_debug& >> - (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) >> - eb |= 1u<< DB_VECTOR; >> if (vcpu->guest_debug& KVM_GUESTDBG_USE_SW_BP) >> eb |= 1u<< BP_VECTOR; >> } >> @@ -3629,7 +3631,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) >> */ >> vmcs_writel(HOST_CR0, read_cr0()); >> >> - set_debugreg(vcpu->arch.dr6, 6); >> + if (vcpu->arch.switch_db_regs) >> + set_debugreg(vcpu->arch.dr6, 6); >> >> asm( >> /* Store host registers */ >> @@ -3731,7 +3734,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) >> | (1<< VCPU_EXREG_PDPTR)); >> vcpu->arch.regs_dirty = 0; >> >> - get_debugreg(vcpu->arch.dr6, 6); >> + if (vcpu->arch.switch_db_regs) >> + get_debugreg(vcpu->arch.dr6, 6); >> >> vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD); >> if (vmx->rmode.irq.pending) >> > ...I wonder why we cannot drop this save/restore? The guest is not able > to access dr6 without causing a trap, thus will never see dr6 as stored > in the hardware. > I think you're right. btw, something else I've considered was not to do any emulation for 'mov dr' but instead load the debug registers and disable the intercept. So far the only issue I've seen is that we lose support for real mode guest self-debug on intel (pre-unrestricted guest). What do you think of this?
Avi Kivity wrote: > On 09/01/2009 04:15 PM, Jan Kiszka wrote: >> Avi Kivity wrote: >> >>> Only reload debug register 6 if we're running with the guest's >>> debug registers. Saves around 150 cycles from the guest lightweight >>> exit path. >>> >>> dr6 contains a couple of bits that are updated on #DB, so intercept >>> that unconditionally and update those bits then. >>> >>> Signed-off-by: Avi Kivity<avi@redhat.com> >>> --- >>> v2: trap #DB so we maintain the TF related bits of DR7. >>> >>> arch/x86/kvm/vmx.c | 14 +++++++++----- >>> 1 files changed, 9 insertions(+), 5 deletions(-) >>> >>> >>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c >>> index 05cd554..d4be978 100644 >>> --- a/arch/x86/kvm/vmx.c >>> +++ b/arch/x86/kvm/vmx.c >>> @@ -540,10 +540,12 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu) >>> eb = (1u<< PF_VECTOR) | (1u<< UD_VECTOR) | (1u<< MC_VECTOR); >>> if (!vcpu->fpu_active) >>> eb |= 1u<< NM_VECTOR; >>> + /* >>> + * Unconditionally intercept #DB so we can maintain dr6 without >>> + * reading it every exit. >>> + */ >>> + eb |= 1u<< DB_VECTOR; >>> >> If this is safe... >> >> >>> if (vcpu->guest_debug& KVM_GUESTDBG_ENABLE) { >>> - if (vcpu->guest_debug& >>> - (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) >>> - eb |= 1u<< DB_VECTOR; >>> if (vcpu->guest_debug& KVM_GUESTDBG_USE_SW_BP) >>> eb |= 1u<< BP_VECTOR; >>> } >>> @@ -3629,7 +3631,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) >>> */ >>> vmcs_writel(HOST_CR0, read_cr0()); >>> >>> - set_debugreg(vcpu->arch.dr6, 6); >>> + if (vcpu->arch.switch_db_regs) >>> + set_debugreg(vcpu->arch.dr6, 6); >>> >>> asm( >>> /* Store host registers */ >>> @@ -3731,7 +3734,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) >>> | (1<< VCPU_EXREG_PDPTR)); >>> vcpu->arch.regs_dirty = 0; >>> >>> - get_debugreg(vcpu->arch.dr6, 6); >>> + if (vcpu->arch.switch_db_regs) >>> + get_debugreg(vcpu->arch.dr6, 6); >>> >>> vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD); >>> if (vmx->rmode.irq.pending) >>> >> ...I wonder why we cannot drop this save/restore? The guest is not able >> to access dr6 without causing a trap, thus will never see dr6 as stored >> in the hardware. >> > > I think you're right. > > btw, something else I've considered was not to do any emulation for 'mov > dr' but instead load the debug registers and disable the intercept. So > far the only issue I've seen is that we lose support for real mode guest > self-debug on intel (pre-unrestricted guest). What do you think of this? I think you can't have both: optimized dr save/restore on vmentry/exit and optimized dr access. If you drop on-demand dr register readout/update, you need to deal with this on every vmentry/exit. My feeling is that this would be awfully slower, even slower than what we currently have without your patches. Jan
On 09/01/2009 04:47 PM, Jan Kiszka wrote: >> btw, something else I've considered was not to do any emulation for 'mov >> dr' but instead load the debug registers and disable the intercept. So >> far the only issue I've seen is that we lose support for real mode guest >> self-debug on intel (pre-unrestricted guest). What do you think of this? >> > I think you can't have both: optimized dr save/restore on vmentry/exit > and optimized dr access. If you drop on-demand dr register > readout/update, you need to deal with this on every vmentry/exit. My > feeling is that this would be awfully slower, even slower than what we > currently have without your patches. > I'll clarify: In the normal case, we'll have #DB and MOV DR intercepted. On #DB, update vcpu->dr6. On MOV DR, disable MOV DR interception, enable dr swap, and let the guest execute; on the next exit, reenable MOV DR interception and disable dr swap. It's similar to the fpu code where we don't emulate fpu instructions. It doesn't really buy us a lot.
On Tue, Sep 01, 2009 at 04:06:25PM +0300, Avi Kivity wrote: > Only reload debug register 6 if we're running with the guest's > debug registers. Saves around 150 cycles from the guest lightweight > exit path. > > dr6 contains a couple of bits that are updated on #DB, so intercept > that unconditionally and update those bits then. Applied, thanks. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Marcelo Tosatti wrote: > On Tue, Sep 01, 2009 at 04:06:25PM +0300, Avi Kivity wrote: >> Only reload debug register 6 if we're running with the guest's >> debug registers. Saves around 150 cycles from the guest lightweight >> exit path. >> >> dr6 contains a couple of bits that are updated on #DB, so intercept >> that unconditionally and update those bits then. > > Applied, thanks. > Will send a cleanup patch based on the discussion with Avi later. It just requires some testing. And I also have a kgdb-safe(r) version of the switch optimization here. Jan
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index 05cd554..d4be978 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -540,10 +540,12 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu) eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR); if (!vcpu->fpu_active) eb |= 1u << NM_VECTOR; + /* + * Unconditionally intercept #DB so we can maintain dr6 without + * reading it every exit. + */ + eb |= 1u << DB_VECTOR; if (vcpu->guest_debug & KVM_GUESTDBG_ENABLE) { - if (vcpu->guest_debug & - (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) - eb |= 1u << DB_VECTOR; if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP) eb |= 1u << BP_VECTOR; } @@ -3629,7 +3631,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) */ vmcs_writel(HOST_CR0, read_cr0()); - set_debugreg(vcpu->arch.dr6, 6); + if (vcpu->arch.switch_db_regs) + set_debugreg(vcpu->arch.dr6, 6); asm( /* Store host registers */ @@ -3731,7 +3734,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) | (1 << VCPU_EXREG_PDPTR)); vcpu->arch.regs_dirty = 0; - get_debugreg(vcpu->arch.dr6, 6); + if (vcpu->arch.switch_db_regs) + get_debugreg(vcpu->arch.dr6, 6); vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD); if (vmx->rmode.irq.pending)
Only reload debug register 6 if we're running with the guest's debug registers. Saves around 150 cycles from the guest lightweight exit path. dr6 contains a couple of bits that are updated on #DB, so intercept that unconditionally and update those bits then. Signed-off-by: Avi Kivity <avi@redhat.com> --- v2: trap #DB so we maintain the TF related bits of DR7. arch/x86/kvm/vmx.c | 14 +++++++++----- 1 files changed, 9 insertions(+), 5 deletions(-)