linux-kernel - Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events

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Message-ID: <4AF08B7C.8040903@web.de>
Date:	Tue, 03 Nov 2009 20:58:52 +0100
From:	Jan Kiszka <jan.kiszka@....de>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jiri Slaby <jirislaby@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Avi Kivity <avi@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Mike Galbraith <efault@....de>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer
 on top of perf events

Frederic Weisbecker wrote:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fc2974a..6560129 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 +3644,12 @@ 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);
> -	}
> +	/*
> +	 * CHECKME: is vcpu->arch.switch_db_regs sufficient to check
> +	 * if the guest is using breakpoints? If so we may want to do
> +	 * this check before.
> +	 */
> +	hw_breakpoint_restore();

Obviously, this variant will make KVM users very unhappy. But trying to
reduce this performance regression via vcpu->arch.switch_db_regs will
make hw-breakpoint users unhappy: KVM leaves at least dr7 clobbered
behind, even if the guest does not use breakpoints.

We really need a replacement for TIF_DEBUG (but we only need this [1]).

Jan

[1]http://thread.gmane.org/gmane.comp.emulators.kvm.devel/39784/focus=39827


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