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From: "Pavel Dovgalyuk" <dovgaluk@ispras.ru>
Date: Tue, 22 Jan 2019 10:15:03 +0300
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Subject: [Qemu-devel] Bottom halves VS timers
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To: qemu-devel@nongnu.org
Cc: pbonzini@redhat.com, peter.maydell@linaro.org, stefanha@redhat.com, famz@redhat.com, kwolf@redhat.com

Hi!

 

I recently debugged record/replay for some platform which uses PIO access for IDE.

Handlers for these operations are called through BHs.

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).

Therefore vCPU may execute 10000000 instruction until BH processing and the guest

software may get a timeout.

 

I thought about replacing BH to the timers with period=0. This solution has the following
properties:

1.       Avoid code duplication (BH and timers do virtually the same operations)

2.       Timers affect the vCPU - adding new virtual timer suspends the execution to process the
callback

3.       Timers are deterministic in record/replay mode (virtual ones of course). We have similar
deterministic concept for BHs, but selecting between virtual and realtime should be more familiar
for other developers.

4.       Timers require a little bit more code, but this could be solved with better interfaces

 

What do you think about this?

Did I miss anything?

 

Pavel Dovgalyuk