[Qemu-devel] Doubts regarding parallelism on KVM, IO threads

[Qemu-devel] Doubts regarding parallelism on KVM, IO threads

[Gaurav Sharma]

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Hi was trying to do some digging for multi core scenarios both with and
without KVM.

In short i have some devices and a user application that does some r/w
operations on those devices.

As per my understanding, in case binary translation using TCG is invoked,
we only create a single Qemuthread for all vcpu's. In case of KVM we have a
Qemuthread for each vcpu as TCG is bypassed in this case.


[Test scenario]
Lets say i have devices dev1 and dev2. Test application for dev1 is
executed on core0 and dev2 on core1.
For device dev1, for testing purposes i specified some sleep whenever a
read comes.
In case KVM is enabled, whenever the sleep is hit my whole VM freezes.

1. Are are devices emulated in a separate single thread ?
2. How and where in the code do we do a switch between CPU thread and
IOthread ?

Regards,
Gaurav

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Re: [Qemu-devel] Doubts regarding parallelism on KVM, IO threads

[Paolo Bonzini]

On 03/03/2016 07:09, Gaurav Sharma wrote:
> Hi was trying to do some digging for multi core scenarios both with and
> without KVM.
> 
> In short i have some devices and a user application that does some r/w
> operations on those devices.
> 
> As per my understanding, in case binary translation using TCG is
> invoked, we only create a single Qemuthread for all vcpu's. In case of
> KVM we have a Qemuthread for each vcpu as TCG is bypassed in this case.
> 
> 
> [Test scenario]
> Lets say i have devices dev1 and dev2. Test application for dev1 is
> executed on core0 and dev2 on core1. 
> For device dev1, for testing purposes i specified some sleep whenever a
> read comes.
> In case KVM is enabled, whenever the sleep is hit my whole VM freezes.
> 
> 1. Are are devices emulated in a separate single thread ?

No, but device emulation and I/O all run behind the same mutex
(affectionately called "big QEMU lock").  See qemu_mutex_lock_iothread
and qemu_mutex_unlock_iothread.

Paolo

> 2. How and where in the code do we do a switch between CPU thread and
> IOthread ?