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Re: [Qemu-devel] MTTCG Tasks (kvmforum summary)

From: Lluís Vilanova
Subject: Re: [Qemu-devel] MTTCG Tasks (kvmforum summary)
Date: Fri, 04 Sep 2015 14:38:49 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.4 (gnu/linux)

dovgaluk  writes:

> Hi!
> Alex Bennée писал 2015-09-04 10:49:
>> * What to do about icount?
>> What is the impact of multi-thread on icount? Do we need to disable it
>> for MTTCG or can it be correct per-cpu? Can it be updated lock-step?

> Why can't we have separate icount for each CPU?
> Then virtual timer will be assigned to one of them.

My understanding is that icount means by deasign that time should be
synchronized between cpus, where the number of executed instructions is the time
unit. If all elements worked under this assumption (I'm afraid that's not the
case for I/O devices), it should be possible to reproduce executions by setting
icount to 1.

Now, MTTCG faces the same icount accuracy problems that the current TCG
implementation deals with (only at different scale). The naive implementation is
to execute 1 instruction per CPU in lockstep. TCG currently relaxes this at the
translation block level.

The MTTCG implementation could do something similar, but just at a different
(configurable?) granularity. Every N per-cpu instructions, synchronize all CPUs
until each has, at least, arrived at that time step, then proceed with the next
batch. Ideally, this synchronization delay (N) could be adapted dynamically.

My half cent.


"And it's much the same thing with knowledge, for whenever you learn
something new, the whole world becomes that much richer."
-- The Princess of Pure Reason, as told by Norton Juster in The Phantom

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