linux-kernel - Re: [PATCH 2/3] MIPS: Setup an instruction emulation in VDSO protected page instead of user stack

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Message-ID: <20141004200016.GB7509@worktop.ger.corp.intel.com>
Date:	Sat, 4 Oct 2014 22:00:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
Cc:	linux-mips@...ux-mips.org, Zubair.Kakakhel@...tec.com,
	david.daney@...ium.com, paul.gortmaker@...driver.com,
	davidlohr@...com, macro@...ux-mips.org, chenhc@...ote.com,
	zajec5@...il.com, james.hogan@...tec.com, keescook@...omium.org,
	alex@...x-smith.me.uk, tglx@...utronix.de, blogic@...nwrt.org,
	jchandra@...adcom.com, paul.burton@...tec.com,
	qais.yousef@...tec.com, linux-kernel@...r.kernel.org,
	ralf@...ux-mips.org, markos.chandras@...tec.com,
	manuel.lauss@...il.com, akpm@...ux-foundation.org,
	lars.persson@...s.com
Subject: Re: [PATCH 2/3] MIPS: Setup an instruction emulation in VDSO
 protected page instead of user stack

On Fri, Oct 03, 2014 at 08:17:30PM -0700, Leonid Yegoshin wrote:

> --- a/arch/mips/include/asm/switch_to.h
> +++ b/arch/mips/include/asm/switch_to.h
> @@ -17,6 +17,7 @@
>  #include <asm/dsp.h>
>  #include <asm/cop2.h>
>  #include <asm/msa.h>
> +#include <asm/mmu_context.h>
>  
>  struct task_struct;
>  
> @@ -104,6 +105,16 @@ do {									\
>  	disable_msa();							\
>  } while (0)
>  
> +static inline void flush_vdso_page(void)
> +{
> +	if (current->mm && cpu_context(raw_smp_processor_id(), current->mm) &&
> +	    (current->mm->context.vdso_asid[raw_smp_processor_id()] ==
> +	     cpu_asid(raw_smp_processor_id(), current->mm))) {
> +		local_flush_tlb_page(current->mm->mmap, (unsigned long)current->mm->context.vdso);
> +		current->mm->context.vdso_asid[raw_smp_processor_id()] = 0;
> +	}
> +}

Why raw_smp_processor_id() and why evaluate it 3 times, sure compilers
can be expected to do some CSE but something like:

	int cpu = smp_processor_id();

	if ( ... [cpu] ...)

is far more readable as well.

> +
>  #define finish_arch_switch(prev)					\
>  do {									\
>  	u32 __c0_stat;							\
> @@ -118,6 +129,7 @@ do {									\
>  		__restore_dsp(current);					\
>  	if (cpu_has_userlocal)						\
>  		write_c0_userlocal(current_thread_info()->tp_value);	\
> +	flush_vdso_page();                                              \
>  	__restore_watch();						\
>  } while (0)

So what I didn't see is any talk about the cost of this. Surely a TLB
flush isn't exactly free.
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