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SubjectRe: [PATCH v3 2/8] powerpc/vdso32: Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE
Michael Ellerman <mpe@ellerman.id.au> a écrit :

> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>> This is copied and adapted from commit 5c929885f1bb ("powerpc/vdso64:
>> Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE")
>> from Santosh Sivaraj <santosh@fossix.org>
>>
>> Benchmark from vdsotest-all:
>> clock-gettime-realtime: syscall: 3601 nsec/call
>> clock-gettime-realtime: libc: 1072 nsec/call
>> clock-gettime-realtime: vdso: 931 nsec/call
>> clock-gettime-monotonic: syscall: 4034 nsec/call
>> clock-gettime-monotonic: libc: 1213 nsec/call
>> clock-gettime-monotonic: vdso: 1076 nsec/call
>> clock-gettime-realtime-coarse: syscall: 2722 nsec/call
>> clock-gettime-realtime-coarse: libc: 805 nsec/call
>> clock-gettime-realtime-coarse: vdso: 668 nsec/call
>> clock-gettime-monotonic-coarse: syscall: 2949 nsec/call
>> clock-gettime-monotonic-coarse: libc: 882 nsec/call
>> clock-gettime-monotonic-coarse: vdso: 745 nsec/call
>>
>> Additional test passed with:
>> vdsotest -d 30 clock-gettime-monotonic-coarse verify
>
> This broke on 64-bit big endian, which uses the 32-bit VDSO, with errors
> like:
>
> clock-gettime-monotonic-coarse/verify: 10 failures/inconsistencies
> encountered
> timestamp obtained from libc/vDSO not normalized:
> [-1574202155, 1061008673]
> timestamp obtained from libc/vDSO predates timestamp
> previously obtained from kernel:
> [74, 261310747] (kernel)
> [-1574202155, 1061008673] (vDSO)
> timestamp obtained from libc/vDSO not normalized:
> [-1574202155, 1061008673]
> timestamp obtained from libc/vDSO predates timestamp
> previously obtained from kernel:
> [74, 261310747] (kernel)
> [-1574202155, 1061008673] (vDSO)
> timestamp obtained from libc/vDSO not normalized:
> [-1574202155, 1061008673]
> timestamp obtained from libc/vDSO predates timestamp
> previously obtained from kernel:
> [74, 261310747] (kernel)
> [-1574202155, 1061008673] (vDSO)
> timestamp obtained from libc/vDSO not normalized:
> [-1574202155, 1061008673]
> timestamp obtained from libc/vDSO predates timestamp
> previously obtained from kernel:
> [74, 261310747] (kernel)
> [-1574202155, 1061008673] (vDSO)
> timestamp obtained from libc/vDSO not normalized:
> [-1574202155, 1061008673]
> timestamp obtained from libc/vDSO predates timestamp
> previously obtained from kernel:
> [74, 261310747] (kernel)
> [-1574202155, 1061008673] (vDSO)
> Failure threshold (10) reached; stopping test.
>
>
> The diff below seems to fix it, but I'm not sure it's correct. ie. we
> just ignore the top part of the values, how does that work?

Your change makes sense, it is consistent with other functions using
STAMP_XTIME.

It works because nanoseconds are max 999999999, it fits 32 bits regs.

Christophe

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