linux-kernel - Re: [PATCH] tools/power turbostat: call pread64 in kernel directly

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Message-ID: <CAJvTdKk7Gd3G4K7um=6HC3YJu48aVW+4WFvr8jVqyOKcSZc7qQ@mail.gmail.com>
Date:   Thu, 3 Sep 2020 13:36:14 -0400
From:   Len Brown <lenb@...nel.org>
To:     Liwei Song <liwei.song@...driver.com>
Cc:     Alexander Monakov <amonakov@...ras.ru>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tools/power turbostat: call pread64 in kernel directly

> -D_FILE_OFFSET_BITS=64

Applied.

thanks!
-Len

On Mon, Aug 24, 2020 at 12:09 AM Liwei Song <liwei.song@...driver.com> wrote:
>
>
>
> On 8/24/20 04:54, Alexander Monakov wrote:
> > Hi,
> >
> > I am not the original submitter, but I have answers and a proper patch :)
> >
> > On Fri, 21 Aug 2020, Len Brown wrote:
> >
> >> Re: offset size
> >>
> >> The offsets on this file are the MSR offsets.
> >> What MSR are you trying to access at offset 0xc0010299?
> >
> > This MSR is particular is part of AMD RAPL (energy measurements) interface.
> >
> >> Re: pread vs pread64
> >>
> >> If I take on faith that you have some kind of 32-bit execution
> >> environment that makes pread into pread32 instead of pread64, and that
> >> truncates an off_t to 32-bits from 64-bits, and it actually makes
> >> sense to request a read at this large offset...
> >
> > The problem here stems from the backward compatibility in Glibc: off_t is
> > 32-bit on 32-bit x86, unless compiled with -D_FILE_OFFSET_BITS=64. This
> > macro should be used for all new code. Distros should enable it for all
> > builds, but when one builds turbostat 'by hand', they hit the issue.
> >
> >> would we really have to invoke syscall() directly -- couldn't we
> >> invoke pread64() directly? (eg. below)
> >
> > No, the proper fix is to pass -D_FILE_OFFSET_BITS=64 to the compiler.
> >
> > Here's the patch:
>
> This path works with my case.
>
> Thanks,
> Liwei.
>
>
> >
> > ---8<---
> >
> > From: Alexander Monakov <amonakov@...ras.ru>
> > Date: Sun, 23 Aug 2020 23:27:02 +0300
> > Subject: [PATCH] turbostat: build with _FILE_OFFSET_BITS=64
> >
> > For compatibility reasons, Glibc off_t is a 32-bit type on 32-bit x86
> > unless _FILE_OFFSET_BITS=64 is defined. Add this define, as otherwise
> > reading MSRs with index 0x80000000 and above attempts a pread with a
> > negative offset, which fails.
> >
> > Signed-off-by: Alexander Monakov <amonakov@...ras.ru>
> > ---
> >  tools/power/x86/turbostat/Makefile | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/tools/power/x86/turbostat/Makefile b/tools/power/x86/turbostat/Makefile
> > index 2b6551269e43..40ae44402eec 100644
> > --- a/tools/power/x86/turbostat/Makefile
> > +++ b/tools/power/x86/turbostat/Makefile
> > @@ -12,6 +12,7 @@ turbostat : turbostat.c
> >  override CFLAGS +=   -O2 -Wall -I../../../include
> >  override CFLAGS +=   -DMSRHEADER='"../../../../arch/x86/include/asm/msr-index.h"'
> >  override CFLAGS +=   -DINTEL_FAMILY_HEADER='"../../../../arch/x86/include/asm/intel-family.h"'
> > +override CFLAGS +=   -D_FILE_OFFSET_BITS=64
> >  override CFLAGS +=   -D_FORTIFY_SOURCE=2
> >
> >  %: %.c
> >



-- 
Len Brown, Intel Open Source Technology Center