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Re: [Qemu-devel] [PATCH v4 3/5] icount: decouple warp calls

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v4 3/5] icount: decouple warp calls
Date: Thu, 10 Mar 2016 14:39:02 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 10/03/2016 14:19, Pavel Dovgalyuk wrote:
>> From: Paolo Bonzini [mailto:address@hidden
>> On 10/03/2016 12:56, Pavel Dovgalyuk wrote:
>>> qemu_clock_warp function is called to update virtual clock when CPU
>>> is sleeping. This function includes replay checkpoint to make execution
>>> deterministic in icount mode.
>>> Record/replay module flushes async event queue at checkpoints.
>>> Some of the events (e.g., block devices operations) include interaction
>>> with hardware. E.g., APIC polled by block devices sets one of IRQ flags.
>>> Flag to be set depends on currently executed thread (CPU or iothread).
>>> Therefore in replay mode we have to process the checkpoints in the same 
>>> thread
>>> as they were recorded.
>>> qemu_clock_warp function (and its checkpoint) may be called from different
>>> thread. This patch decouples two different execution cases of this function:
>>> call when CPU is sleeping from iothread and call from cpu thread to update
>>> virtual clock.
>>> First task is performed by qemu_start_warp_timer function. It sets warp
>>> timer event to the moment of nearest pending virtual timer.
>>> Second function (qemu_account_warp_timer) is called from cpu thread
>>> before execution of the code. It advances virtual clock by adding the length
>>> of period while CPU was sleeping.
>>> Signed-off-by: Pavel Dovgalyuk <address@hidden>
>> Lovely. :)  One question, why doesn't icount_dummy_timer need a checkpoint?
> It is synchronized with CHECKPOINT_CLOCK_VIRTUAL_RT.
>> Only needs a change to the documentation:
> Ok, I'll change it.

No problem, I can do it.


>> diff --git a/docs/replay.txt b/docs/replay.txt
>> index 149727e..26dfb6e 100644
>> --- a/docs/replay.txt
>> +++ b/docs/replay.txt
>> @@ -134,11 +134,18 @@ of time. That's why we do not process a group of 
>> timers until the
>> checkpoint
>>  event will be read from the log. Such an event allows synchronizing CPU
>>  execution and timer events.
>> -Another checkpoints application in record/replay is instruction counting
>> -while the virtual machine is idle. This function (qemu_clock_warp) is called
>> -from the wait loop. It changes virtual machine state and must be 
>> deterministic
>> -then. That is why we added checkpoint to this function to prevent its
>> -operation in replay mode when it does not correspond to record mode.
>> +Two other checkpoints govern the "warping" of the virtual clock.  While
>> +the virtual machine is idle, the virtual clock increments at 1 ns per
>> +*real time* nanosecond.  This is done by setting up a timer (called the
>> +warp timer) and then incrementing the virtual clock (called "warping"
>> +the virtual clock) as soon as the CPUs need to go out of the idle state.
>> +These actions change virtual machine state and must be deterministic.
>> +Two functions are used for this purpose, and each of them creates a
>> +checkpoint.  qemu_start_warp_timer checks if the CPUs are idle and if so
>> +starts accounting real time to virtual clock.  qemu_account_warp_timer
>> +is called when the CPUs get an interrupt or when a virtual clock timer
>> +fires, and it warps the virtual clock by the amount of real time that
>> +has passed since qemu_start_warp_timer.
> Pavel Dovgalyuk

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