linux-kernel - Re: [patch 05/24] perfmon: X86 generic code (x86)

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Message-Id: <20081127.033742.175440395.davem@davemloft.net>
Date:	Thu, 27 Nov 2008 03:37:42 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	tglx@...utronix.de
Cc:	eranian@...il.com, sfr@...b.auug.org.au, andi@...stfloor.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, x86@...nel.org
Subject: Re: [patch 05/24] perfmon: X86 generic code (x86)

From: Thomas Gleixner <tglx@...utronix.de>
Date: Thu, 27 Nov 2008 11:56:24 +0100 (CET)

> Stephane,
> 
> On Thu, 27 Nov 2008, stephane eranian wrote:
> > > Why do you want to do u64 -> u32 BE32 magic on every interrupt,
> > > context switch etc., if you can do it once in the userspace interface ?
> > >
> > I think we agree as to why the user visible structures have to have fixed size.
> > 
> > Some structures have bitfields in them (no in the current patchset yet).
> > Here is an example:
> > 
> > #define PFM_PMD_BV   (256/sizeof(__u64))
> > 
> > struct pfarg_pmd_attr {
> >         __u16 reg_num;          /* which register */
> >         __u16 reg_set;          /* which event set */
> >         __u32 reg_flags;        /* REGFL flags */
> >         __u64 reg_value;        /* 64-bit value */
> >         __u64 reg_long_reset;   /* write: value to reload after notification */
> >         __u64 reg_short_reset;  /* write: reset after counter overflow */
> >         __u64 reg_random_mask;  /* write: bitmask used to limit random value */
> >         __u64 reg_smpl_pmds[PFM_PMD_BV];  /* write: record in sample */
> >         __u64 reg_reset_pmds[PFM_PMD_BV]; /* write: reset on overflow */
> >         __u64 reg_ovfl_swcnt;   /* write: # of overflows before switch */
> >         __u64 reg_smpl_eventid; /* write: opaque event identifier */
> >         __u64 reg_last_value;   /* read : return: PMD last reset value */
> >         __u64 reg_reserved[8];  /* for future use */
> > };
> 
>   __attribute__ ((packed)); ???

Shouldn't be needed and this woule kill performance on RISC (every
struct member is accessed with byte sized loads and shifts/masking).

We did this for the pfkey.h structures by accident, and I truly regret
it.
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