linux-kernel - Re: [patch 06/11] Use virtual debug registers in process/thread handling code

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Date:	Tue, 10 Mar 2009 15:49:33 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	prasad@...ux.vnet.ibm.com
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [patch 06/11] Use virtual debug registers in process/thread
	handling code


* prasad@...ux.vnet.ibm.com <prasad@...ux.vnet.ibm.com> wrote:

> @@ -437,16 +448,6 @@ __switch_to_xtra(struct task_struct *pre
>  	else if (next->debugctlmsr != prev->debugctlmsr)
>  		update_debugctlmsr(next->debugctlmsr);
>  
> -	if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
> -		set_debugreg(next->debugreg0, 0);
> -		set_debugreg(next->debugreg1, 1);
> -		set_debugreg(next->debugreg2, 2);
> -		set_debugreg(next->debugreg3, 3);
> -		/* no 4 and 5 */
> -		set_debugreg(next->debugreg6, 6);
> -		set_debugreg(next->debugreg7, 7);
> -	}
> -
>  	if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
>  	    test_tsk_thread_flag(next_p, TIF_NOTSC)) {
>  		/* prev and next are different */
> @@ -595,6 +596,12 @@ __switch_to(struct task_struct *prev_p, 
>  
>  	percpu_write(current_task, next_p);
>  
> +	/*
> +	 * Handle debug registers.  This must be done _after_ current
> +	 * is updated.
> +	 */
> +	if (unlikely(test_tsk_thread_flag(next_p, TIF_DEBUG)))
> +		switch_to_thread_hw_breakpoint(next_p);

why does this have to be called after 'current' has been 
updated? AFAICS switch_to_thread_hw_breakpoint() does not take a 
look at 'current'.

Speaking of switch_to_thread_hw_breakpoint(), i dont like that 
function at all:

- why does it have to do a list of debug registers?

- why does it worry about IPIs arriving when context-switches on 
  x86 are always done with interrupts disabled?

- also, what do the ->installed() and ->uninstalled() callbacks 
  do - nothing uses it!

	Ingo
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