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Re: [Qemu-devel] Bottom halves VS timers

From: Paolo Bonzini
Subject: Re: [Qemu-devel] Bottom halves VS timers
Date: Tue, 22 Jan 2019 09:49:58 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

On 22/01/19 08:15, Pavel Dovgalyuk wrote:
> Scheduling new BH does not create a signal for the vCPU to suspend the
> execution and process the main loop events (in record/replay we
> synchronize these two threads).

I think the bug is that in rr mode it should do so and cause a cpu_exit.
 If replace these bottom halves with timers, other bottom halves from
the block layer may have the same issue; if you replace _all_ bottom
halves with timers you have performance issues because bottom halves try
very hard to avoid locks.

Unfortunately this is getting really ugly...  Basically, rr is
reintroducing all the complications with iothread_requesting_mutex that
were removed with mtTCG.


> Therefore vCPU may execute 10000000 instruction until BH processing and
> the guest
> software may get a timeout.

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