linux-kernel - Re: [PATCH v6 1/3] x86/fpu: Measure the Latency of XSAVES and XRSTORS

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Message-ID: <ZPMTVNM2oBCdSYjJ@gmail.com>
Date:   Sat, 2 Sep 2023 12:49:56 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Yi Sun <yi.sun@...el.com>
Cc:     dave.hansen@...el.com, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        sohil.mehta@...el.com, ak@...ux.intel.com,
        ilpo.jarvinen@...ux.intel.com, heng.su@...el.com,
        tony.luck@...el.com, dave.hansen@...ux.intel.com,
        yi.sun@...el.intel.com
Subject: Re: [PATCH v6 1/3] x86/fpu: Measure the Latency of XSAVES and XRSTORS


* Yi Sun <yi.sun@...el.com> wrote:

> +#define XSTATE_XSAVE(fps, lmask, hmask, err)				\
> +	do {								\
> +		struct fpstate *f = fps;				\
> +		u64 tc = -1;						\
> +		if (xsave_tracing_enabled())				\
> +			tc = trace_clock();				\
> +		__XSTATE_XSAVE(&f->regs.xsave, lmask, hmask, err);	\
> +		if (xsave_tracing_enabled())				\
> +			trace_x86_fpu_latency_xsave(f, trace_clock() - tc);\
> +	} while (0)
> +
>  /*
>   * Use XRSTORS to restore context if it is enabled. XRSTORS supports compact
>   * XSAVE area format.
>   */
> -#define XSTATE_XRESTORE(st, lmask, hmask)				\
> +#define __XSTATE_XRESTORE(st, lmask, hmask)				\
>  	asm volatile(ALTERNATIVE(XRSTOR,				\
>  				 XRSTORS, X86_FEATURE_XSAVES)		\
>  		     "\n"						\
> @@ -140,6 +168,17 @@ static inline u64 xfeatures_mask_independent(void)
>  		     : "D" (st), "m" (*st), "a" (lmask), "d" (hmask)	\
>  		     : "memory")
>  
> +#define XSTATE_XRESTORE(fps, lmask, hmask)				\
> +	do {								\
> +		struct fpstate *f = fps;				\
> +		u64 tc = -1;						\
> +		if (xrstor_tracing_enabled())				\
> +			tc = trace_clock();				\
> +		__XSTATE_XRESTORE(&f->regs.xsave, lmask, hmask);	\
> +		if (xrstor_tracing_enabled())				\
> +			trace_x86_fpu_latency_xrstor(f, trace_clock() - tc);\
> +	} while (0)
> +
>  #if defined(CONFIG_X86_64) && defined(CONFIG_X86_DEBUG_FPU)
>  extern void xfd_validate_state(struct fpstate *fpstate, u64 mask, bool rstor);
>  #else
> @@ -184,7 +223,7 @@ static inline void os_xsave(struct fpstate *fpstate)
>  	WARN_ON_FPU(!alternatives_patched);
>  	xfd_validate_state(fpstate, mask, false);
>  
> -	XSTATE_XSAVE(&fpstate->regs.xsave, lmask, hmask, err);
> +	XSTATE_XSAVE(fpstate, lmask, hmask, err);
>  
>  	/* We should never fault when copying to a kernel buffer: */
>  	WARN_ON_FPU(err);
> @@ -201,7 +240,7 @@ static inline void os_xrstor(struct fpstate *fpstate, u64 mask)
>  	u32 hmask = mask >> 32;
>  
>  	xfd_validate_state(fpstate, mask, true);
> -	XSTATE_XRESTORE(&fpstate->regs.xsave, lmask, hmask);
> +	XSTATE_XRESTORE(fpstate, lmask, hmask);
>  }

Instead of adding overhead to the regular FPU context saving/restoring code 
paths, could you add a helper function that has tracing code included, but 
which isn't otherwise used - and leave the regular code with no tracing 
overhead?

This puts a bit of a long-term maintenance focus on making sure that the 
traced functionality won't bitrot, but I'd say that's preferable to adding 
tracing overhead.

Thanks,

	Ingo