From: Paolo Bonzini <pbonzini@redhat.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: QEMU Developers <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [PATCH] icount: disable icount with multiprocessor guests
Date: Fri, 27 Sep 2013 10:23:12 +0200 [thread overview]
Message-ID: <52454070.5080502@redhat.com> (raw)
In-Reply-To: <CAFEAcA_OKcmksDVRgGFPZtSxNbnU2vj=yrVkYEkztXTKch1enQ@mail.gmail.com>
Il 25/09/2013 01:39, Peter Maydell ha scritto:
> On 25 September 2013 00:52, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> If -icount is enabled with multiprocessor guests, all CPUs increment
>> the same counter, which then basically runs too fast by a factor of
>> smp_cpus. This makes little sense and complicates further a feature
>> whose implementation is pretty obscure already.
>
> Just forbidding -icount with SMP configs seems like the wrong
> fix, though -- why wouldn't you want the deterministic execution
> icount (claims it) gives you with an SMP config?
Right now, the SMP version of icount is all but deterministic. I/O and
rt_clock timers cause the TCG thread to relinquish the BQL, and the
round-robin will switch to the next VCPU. Perhaps once the TCG loop
breaks free of the BQL we can modify icount to trigger a VCPU switch
every 2^16 instructions or something like that, and revert this patch.
Paolo
prev parent reply other threads:[~2013-09-27 8:23 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-24 15:52 [Qemu-devel] [PATCH] icount: disable icount with multiprocessor guests Paolo Bonzini
2013-09-24 23:39 ` Peter Maydell
2013-09-27 8:23 ` Paolo Bonzini [this message]
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