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Re: [Qemu-devel] [PATCH] icount: disable icount with multiprocessor gues


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH] icount: disable icount with multiprocessor guests
Date: Fri, 27 Sep 2013 10:23:12 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130805 Thunderbird/17.0.8

Il 25/09/2013 01:39, Peter Maydell ha scritto:
> On 25 September 2013 00:52, Paolo Bonzini <address@hidden> 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



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