linux-kernel - Re: [PATCH] tools/perf: fix the word selected in find_*

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Date:	Thu, 16 Jun 2016 00:11:04 +0300
From:	Yury Norov <ynorov@...iumnetworks.com>
To:	Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
CC:	<linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Borislav Petkov <bp@...e.de>, David Ahern <dsahern@...il.com>,
	George Spelvin <linux@...izon.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Wang Nan <wangnan0@...wei.com>,
	Yury Norov <yury.norov@...il.com>,
	Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH] tools/perf: fix the word selected in find_*_bit

On Wed, Jun 15, 2016 at 10:51:27PM +0300, Yury Norov wrote:
> Hi Madhavan,
> 
> On Wed, Jun 15, 2016 at 05:12:53PM +0530, Madhavan Srinivasan wrote:
> > When decoding the perf_regs mask in regs_dump__printf(),
> > we loop through the mask using find_first_bit and find_next_bit functions.
> > And mask is of type "u64". But "u64" is send as a "unsigned long *" to
> > lib functions along with sizeof().
> > 
> > While the exisitng code works fine in most of the case, when using a 32bit perf
> > on a 64bit kernel (Big Endian), we end reading the wrong word. In find_first_bit(),
> > one word at a time (based on BITS_PER_LONG) is loaded and
> > checked for any bit set. In 32bit BE userspace,
> > BITS_PER_LONG turns out to be 32, and for a mask value of
> > "0x00000000000000ff", find_first_bit will return 32, instead of 0.
> > Reason for this is that, value in the word0 is all zeros and value
> > in word1 is 0xff. Ideally, second word in the mask should be loaded
> > and searched. Patch swaps the word to look incase of 32bit BE.
> 
> I think this is not a problem of find_bit() at all. You have wrong
> typecast as the source of problem (tools/perf/util/session.c"):
> 
> 940 static void regs_dump__printf(u64 mask, u64 *regs)
> 941 {
> 942         unsigned rid, i = 0;
> 943 
> 944         for_each_set_bit(rid, (unsigned long *) &mask, sizeof(mask) * 8) {
>                                           ^^^^ Here ^^^^
> 945                 u64 val = regs[i++];
> 946 
> 947                 printf(".... %-5s 0x%" PRIx64 "\n",
> 948                        perf_reg_name(rid), val);
> 949         }
> 950 }
> 
> But for some reason you change correct find_bit()...
> 
> Though proper fix is like this for me:
> 
> static void regs_dump__printf(u64 mask, u64 *regs)
> {
>         unsigned rid, i = 0;
>         unsigned long _mask[sizeof(mask)/sizeof(unsigned long)];
> 
>         _mask[0] = mask & ULONG_MAX;
>         if (sizeof(mask) > sizeof(unsigned long))
>                 _mask[1] = mask >> BITS_PER_LONG;
> 
>         for_each_set_bit(rid, _mask, sizeof(mask) * BITS_PER_BYTE) {
>                 u64 val = regs[i++];
> 
>                 printf(".... %-5s 0x%" PRIx64 "\n",
>                        perf_reg_name(rid), val);
>         }
> }
> 
> Maybe there already is some macro doing the conversion for you...

yes it is, cpu_to_le64() is what you want

> 
> Yury.
> 
> > Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> > Cc: Adrian Hunter <adrian.hunter@...el.com>
> > Cc: Borislav Petkov <bp@...e.de>
> > Cc: David Ahern <dsahern@...il.com>
> > Cc: George Spelvin <linux@...izon.com>
> > Cc: Jiri Olsa <jolsa@...hat.com>
> > Cc: Namhyung Kim <namhyung@...nel.org>
> > Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
> > Cc: Wang Nan <wangnan0@...wei.com>
> > Cc: Yury Norov <yury.norov@...il.com>
> > Cc: Michael Ellerman <mpe@...erman.id.au>
> > Signed-off-by: Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
> > ---
> >  tools/lib/find_bit.c | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> > 
> > diff --git a/tools/lib/find_bit.c b/tools/lib/find_bit.c
> > index 9122a9e80046..996b3e04324f 100644
> > --- a/tools/lib/find_bit.c
> > +++ b/tools/lib/find_bit.c
> > @@ -37,7 +37,12 @@ static unsigned long _find_next_bit(const unsigned long *addr,
> >  	if (!nbits || start >= nbits)
> >  		return nbits;
> > 
> > +#if (__BYTE_ORDER == __BIG_ENDIAN) && (BITS_PER_LONG != 64)
> > +	tmp = addr[(((nbits - 1)/BITS_PER_LONG) - (start / BITS_PER_LONG))]
> > +								^ invert;
> > +#else
> >  	tmp = addr[start / BITS_PER_LONG] ^ invert;
> > +#endif
> > 
> >  	/* Handle 1st word. */
> >  	tmp &= BITMAP_FIRST_WORD_MASK(start);
> > @@ -48,7 +53,12 @@ static unsigned long _find_next_bit(const unsigned long *addr,
> >  		if (start >= nbits)
> >  			return nbits;
> > 
> > +#if (__BYTE_ORDER == __BIG_ENDIAN) && (BITS_PER_LONG != 64)
> > +		tmp = addr[(((nbits - 1)/BITS_PER_LONG) - (start / BITS_PER_LONG))]
> > +								^ invert;
> > +#else
> >  		tmp = addr[start / BITS_PER_LONG] ^ invert;
> > +#endif
> >  	}
> > 
> >  	return min(start + __ffs(tmp), nbits);
> > @@ -75,8 +85,15 @@ unsigned long find_first_bit(const unsigned long *addr, unsigned long size)
> >  	unsigned long idx;
> > 
> >  	for (idx = 0; idx * BITS_PER_LONG < size; idx++) {
> > +#if (__BYTE_ORDER == __BIG_ENDIAN) && (BITS_PER_LONG != 64)
> > +		if (addr[(((size-1)/BITS_PER_LONG) - idx)])
> > +			return min(idx * BITS_PER_LONG +
> > +				__ffs(addr[(((size-1)/BITS_PER_LONG) - idx)]),
> > +									size);
> > +#else
> >  		if (addr[idx])
> >  			return min(idx * BITS_PER_LONG + __ffs(addr[idx]), size);
> > +#endif
> >  	}
> > 
> >  	return size;
> > --
> > 1.9.1