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Re: [Qemu-discuss] qemu SMP support

From: Pranith Kumar
Subject: Re: [Qemu-discuss] qemu SMP support
Date: Fri, 17 Oct 2014 12:43:14 -0400

(adding list)

Hi Peter,

On Thu, Oct 16, 2014 at 6:28 PM, Peter Maydell <address@hidden> wrote:
>> I've learnt that multi-threaded execution is not supported in user
>> mode and in system mode with TCG. Is KVM the only mode which supports
>> multi-threaded execution? Or is the support related to SMP mode?
> TCG system mode supports running multi-threaded guests fine,
> and we support both single CPU and multiple CPU guest configs.
> What we don't do is make use of multiple host threads when
> we're doing this, so we emulate a 4 core system by running
> guest instructions for each core in turn within a single host
> thread. This is a performance limitation, not a functional one.
> (If you care largely about performance then for TCG you are
> best off specifying a single guest CPU config; multiple guest
> CPUs will run slower, not faster.)

Thank you for the information. I will use a single guest CPU config
since all I am interested in is the number of instructions.

Is there any way I could communicate from the guest side to qemu TCG
indicating that I want to start counting instructions?

I do not want to count the instructions during boot etc., I want to
communicate to qemu just before launching the user program to flush
the tb cache, start counting and then after the user program is done
running to stop counting and dump the values.

On IRC, some one suggested to add a new system register to ARMRegInfo
and then use mcr, mrc to write to that register to indicate to qemu.
Does this sound OK? Or is there a better way?


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