Frederic Weisbecker wrote:
[...]
> @@ -94,6 +87,12 @@ static inline void hw_breakpoint_disable(void)
>  	set_debugreg(0UL, 3);
>  }
>
> +#ifdef CONFIG_KVM
> +extern void hw_breakpoint_restore(void);
> +#else
> +static inline void hw_breakpoint_restore(void)	{ }
> +#endif
> +

Hmm, if this shall become a KVM-only API, why the empty wrapper?

[...]
> -void arch_flush_thread_hw_breakpoint(struct task_struct *tsk)
> +#ifdef CONFIG_KVM
> +void hw_breakpoint_restore(void)
>  {
> -	int i;
> -	struct thread_struct *thread = &(tsk->thread);
> -
> -	thread->debugreg7 = 0;
> -	for (i = 0; i < HBP_NUM; i++)
> -		thread->debugreg[i] = 0;
> +	set_debugreg(__get_cpu_var(cpu_debugreg[0]), 0);
> +	set_debugreg(__get_cpu_var(cpu_debugreg[1]), 1);
> +	set_debugreg(__get_cpu_var(cpu_debugreg[2]), 2);
> +	set_debugreg(__get_cpu_var(cpu_debugreg[3]), 3);
> +	set_debugreg(current->thread.debugreg6, 6);
> +	set_debugreg(__get_cpu_var(dr7), 7);
>  }
> +#endif

EXPORT_SYMBOL_GPL(hw_breakpoint_restore);

as KVM can be built as module.

[...]
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fc2974a..32d7bca 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -42,6 +42,7 @@
>  #define CREATE_TRACE_POINTS
>  #include "trace.h"
>  
> +#include <asm/debugreg.h>
>  #include <asm/uaccess.h>
>  #include <asm/msr.h>
>  #include <asm/desc.h>
> @@ -3643,14 +3644,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>  	trace_kvm_entry(vcpu->vcpu_id);
>  	kvm_x86_ops->run(vcpu, kvm_run);
>  
> -	if (unlikely(vcpu->arch.switch_db_regs || test_thread_flag(TIF_DEBUG))) {
> -		set_debugreg(current->thread.debugreg[0], 0);
> -		set_debugreg(current->thread.debugreg[1], 1);
> -		set_debugreg(current->thread.debugreg[2], 2);
> -		set_debugreg(current->thread.debugreg[3], 3);
> -		set_debugreg(current->thread.debugreg6, 6);
> -		set_debugreg(current->thread.debugreg7, 7);
> -	}
> +	if (unlikely(vcpu->arch.switch_db_regs || test_thread_flag(TIF_DEBUG)))
> +		hw_breakpoint_restore();

TIF_DEBUG is only set on active ptrace hw-breakpoints, thus we miss
other types here, right? (Note: arch.switch_db_regs is guest-related,
thus does not help in this regard.)

Jan