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From: Peter Maydell <peter.maydell@linaro.org>
Date: Tue, 8 Jul 2014 09:05:10 +0100
Message-ID: <CAFEAcA8_iWUNMojsf5S5Qr+TuC0AbgVuOvL3=vgNz3yLNAfOhg@mail.gmail.com>
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Subject: Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions
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To: Al Viro <viro@zeniv.linux.org.uk>
Cc: QEMU Developers <qemu-devel@nongnu.org>, Richard Henderson <rth@twiddle.net>

On 8 July 2014 08:13, Al Viro <viro@zeniv.linux.org.uk> wrote:
> Actually, that's badly worded; what codepath ends up setting si_code on
> e.g. fp addition overflows?  In system mode it's done by completion code
> in the kernel, but AFAICS in user mode there are only two places where it
> might happen - one is gentrap handling and another - osf_setsysinfo(2)
> emulation for TARGET_SSI_IEEE_FP_CONTROL.  What I don't understand is how
> do we get from float_raise(&FP_STATUS, float_flag_overflow) in fpu/softfloat.c
> to either of those.
>
> IOW, suppose I do
>         x = DBL_MAX;
>         feenableexcept(FE_ALL_EXCEPT);
>         x *= x;
> I understand how I'll get SIGFPE, but what will set correct si_code in
> siginfo I'll see in the hanler?

The code we have currently may well be buggy, but the correct
place to set si_code is (as Richard says) the Alpha cpu_loop() in
linux-user/main.c, which has access to the trap type that just
caused us to stop executing code, plus the CPUState, which
should be enough information to set si_code correctly. In
particular the GENTRAP case seems to be setting a variety
of different si_code values for SIGFPE.

thanks
-- PMM