linux-kernel - Re: [PATCH v3 11/12] selftests/nolibc: add new gettimeofday test cases

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Date:   Sun, 04 Jun 2023 11:24:39 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Zhangjin Wu" <falcon@...ylab.org>,
        Thomas Weißschuh <thomas@...ch.de>
Cc:     "Willy Tarreau" <w@....eu>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v3 11/12] selftests/nolibc: add new gettimeofday test cases

On Sun, Jun 4, 2023, at 10:29, 吴章金 wrote:
>
> Sorry for missing part of your feedbacks, I will check if -nostdlib 
> stops the linking of libgcc_s or my own separated test script forgot 
> linking the libgcc_s manually.

According to the gcc documentation, -nostdlib drops libgcc.a, but
adding -lgcc is the recommended way to bring it back.

> And as suggestion from Thomas' reply,
>
>>> Perhaps we really need to add the missing __divdi3 and __aeabi_ldivmod and the
>>> ones for the other architectures, or get one from lib/math/div64.c.
>
>>No, these ones come from the compiler via libgcc_s, we must not try to
> reimplement them. And we should do our best to avoid depending on them
> to avoid the error you got above.
>
> So, the explicit conversion is used instead in the patch.

I think a cast to a 32-bit type is ideal when converting the
clock_gettime() result into microseconds, since the kernel guarantees
that the timespec value is normalized, with all zeroes in the
upper 34 bits. Going through __aeabi_ldivmod would make the
conversion much slower.

For user supplied non-normalized timeval values, it's not obvious
whether we need the full 64-bit division

     Arnd