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Re: [Qemu-devel] How to break cpu_tb_exec()?

From: Richard Henderson
Subject: Re: [Qemu-devel] How to break cpu_tb_exec()?
Date: Tue, 21 Jul 2015 08:02:04 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0

On 07/06/2015 12:26 PM, Lluís Vilanova wrote:
Jun Koi writes:

On Sat, Jul 4, 2015 at 1:12 AM, Peter Maydell <address@hidden> wrote:
     On 3 July 2015 at 18:10, Jun Koi <address@hidden> wrote:
On Sat, Jul 4, 2015 at 1:06 AM, Peter Maydell <address@hidden>
On 3 July 2015 at 18:02, Jun Koi <address@hidden> wrote:
If this is true, then what if this TB is running infinitely, and do not
return, or it is in a very long loop? In this case, TCG thread cannot

Every TB starts with a little bit of generated code that checks
the 'tcg_exit_req' flag in the CPUState for the CPU (see the
gen_tb_start() function). If some other part of QEMU wants the
CPU to stop running guest code and return to the top level loop,
it calls cpu_exit() which sets this flag.

But this does not answer my question yet: if we the flag is only
enable when TB already enters the "long loop", then nothing can break
this TB execution?

     We check the flag for every TB we execute. Therefore in any
     loop we must check the flag each time round the loop. So
     if another thread sets the flag, we will exit.

     (A TB is always ended by any kind of branch instruction,
     so you can't have a loop within a single TB. A tight loop
     turns into a TB that ends with "branch back to the start
     of this TB", but that will re-execute the flag-check code.)

Oh right, this tight loop is my main concern. It makes sense now.

I'm not sure if "rep"-style x86 instructions loop inside the same TB, though.

They do not.


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