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[Qemu-devel] Bottom halves VS timers

From: Pavel Dovgalyuk
Subject: [Qemu-devel] Bottom halves VS timers
Date: Tue, 22 Jan 2019 10:15:03 +0300



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

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 

Therefore vCPU may execute 10000000 instruction until BH processing and the 

software may get a timeout.


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

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

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


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