Frederic Weisbecker wrote:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fc2974a..22dee7a 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 +3644,15 @@ 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 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
> +	 * care about the messed up debug address registers. But if
> +	 * we have some of them active, restore the old state.
> +	 */
> +	if (__get_cpu_var(dr7) & DR_GLOBAL_ENABLE_MASK)

Looks good, just a minor remark: Would be cleaner to wrap this into an
inline function, say hw_breakpoint_active(), to abstract the precise
condition away from KVM.

> +		hw_breakpoint_restore();
>  
>  	set_bit(KVM_REQ_KICK, &vcpu->requests);
>  	local_irq_enable();

Will see that I can give your series a try the next days, probably
debugging qemu-kvm while running a guest that uses breakpoints. But I
don't expect surprises.

Jan