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

[Liwei Song]

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@xxxxxxxxx>
> 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@xxxxxxxxx>
> ---
>  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
>