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

[Alexander Monakov]

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:

---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
-- 
2.26.2