Re: [PATCH 3/7] KVM: x86: Allow the guest to run with dirty debug registers

Radim Krčmář
2014-03-09T18:28:25Z

2014-03-07 12:42+0100, Paolo Bonzini:
> When not running in guest-debug mode, the guest controls the debug
> registers and having to take an exit for each DR access is a waste
> of time. If the guest gets into a state where each context switch
> causes DR to be saved and restored, this can take away as much as 40%
> of the execution time from the guest.
>
> After this patch, VMX- and SVM-specific code can set a flag in
> switch_db_regs, telling vcpu_enter_guest that on the next exit the debug
> registers might be dirty and need to be reloaded (syncing will be taken
> care of by a new callback in kvm_x86_ops). This flag can be set on the
> first access to a debug registers, so that multiple accesses to the
> debug registers only cause one vmexit.
>
> Note that since the guest will be able to read debug registers and
> enable breakpoints in DR7, we need to ensure that they are synchronized
> on entry to the guest---including DR6 that was not synced before.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/include/asm/kvm_host.h | 2 ++
> arch/x86/kvm/x86.c | 16 ++++++++++++++++
> 2 files changed, 18 insertions(+)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 5ef59d3b6c63..74eb361eaa8f 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -339,6 +339,7 @@ struct kvm_pmu {
>
> enum {
> KVM_DEBUGREG_BP_ENABLED = 1,
> + KVM_DEBUGREG_WONT_EXIT = 2,
> };
>
> struct kvm_vcpu_arch {
> @@ -707,6 +708,7 @@ struct kvm_x86_ops {
> void (*set_gdt)(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
> u64 (*get_dr6)(struct kvm_vcpu *vcpu);
> void (*set_dr6)(struct kvm_vcpu *vcpu, unsigned long value);
> + void (*sync_dirty_debug_regs)(struct kvm_vcpu *vcpu);
> void (*set_dr7)(struct kvm_vcpu *vcpu, unsigned long value);
> void (*cache_reg)(struct kvm_vcpu *vcpu, enum kvm_reg reg);
> unsigned long (*get_rflags)(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 252b47e85c69..c48818aa04c0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6033,12 +6033,28 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> set_debugreg(vcpu->arch.eff_db[1], 1);
> set_debugreg(vcpu->arch.eff_db[2], 2);
> set_debugreg(vcpu->arch.eff_db[3], 3);
> + set_debugreg(vcpu->arch.dr6, 6);
> }
>
> trace_kvm_entry(vcpu->vcpu_id);
> kvm_x86_ops->run(vcpu);
>
> /*
> + * Do this here before restoring debug registers on the host. And
> + * since we do this before handling the vmexit, a DR access vmexit
> + * can (a) read the correct value of the debug registers, (b) set
> + * KVM_DEBUGREG_WONT_EXIT again.

We re-enable intercepts on the next exit for the sake of simplicity?
(Batched accesses make perfect sense, but it seems we don't have to care
about DRs at all, without guest-debug.)

> + */
> + if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)) {
> + int i;
> +
> + WARN_ON(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP);

Is this possible? (I presumed that we vmexit before handling ioctls.)

> + kvm_x86_ops->sync_dirty_debug_regs(vcpu);

Sneaky functionality ... it would be nicer to split 'enable DR
intercepts' into a new kvm op.
I think we want to disable them whenever we are not in guest-debug mode
anyway, so it would be a pair.
And we don't have to modify DR intercepts then, which is probably the
main reason why sync_dirty_debug_regs() does two things.

> + for (i = 0; i < KVM_NR_DB_REGS; i++)
> + vcpu->arch.eff_db[i] = vcpu->arch.db[i];
> + }
> +
> + /*
> * If the guest has used debug registers, at least dr7
> * will be disabled while returning to the host.
> * If we don't have active breakpoints in the host, we don't
> --
> 1.8.3.1
>
>
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