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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <5640A236.4010501@redhat.com>
Date: Mon, 9 Nov 2015 14:40:06 +0100
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Subject: Re: [Qemu-devel] Qemu: Guest Linux hangs on Mac OS X 10.11
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Aaron Elkins <threcius@yahoo.com>, "qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>, "Michael S. Tsirkin" <mst@redhat.com>



On 09/11/2015 14:30, Peter Maydell wrote:
> After a bunch of "try building specific object files with optimization
> off to see where the problem goes away" tests, I've narrowed the
> problem down further: if you tell clang to disable optimization by
> adding __attribute__ ((optnone)) to the two functions
> hpet_time_after() and hpet_time_after64() in hw/timer/hpet.c then
> the problem goes away.
> 
> My current theory is that we're doing something here that's not
> valid C and the compiler ends up optimizing it into something
> that results in the timer setting the next-timeout to a very
> short interval and that's what's hogging the main-loop time.
> I'll look in more detail after lunch.

The obvious way to write those functions would be

static uint32_t hpet_time_after(uint64_t a, uint64_t b)
{
    return ((int32_t)(b - a)) < 0;
}

static uint32_t hpet_time_after64(uint64_t a, uint64_t b)
{
    return ((int64_t)(b - a)) < 0;
}

Paolo