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.
>
> Signed-off-by: Avi Kivity <[email protected]>
> ---
> 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);
> -
I'm fine with this for the common case, but we should switch to the safe
pattern on test_thread_flag(TIF_DEBUG), ie. if someone debugs qemu using
hardware break/watchpoints.
> 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)))
> {
IIRC, you must clear dr7 before loading dr0..3 to avoid spurious effects.
> + 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);
Unless I miss something ATM, moving this into the if-clause should cause
troubles if the guest leaves dr7 armed behind. But I need to refresh my
memory on this.
>
> set_bit(KVM_REQ_KICK, &vcpu->requests);
> local_irq_enable();
Jan
--
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux
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