Re: qemu_coroutine_yield switches thread?

[Kevin Wolf <kwolf@redhat.com>]

Am 16.04.2020 um 10:06 hat Stefan Reiter geschrieben:
> Hi list,
> 
> quick question: Can a resume from a qemu_coroutine_yield happen in a
> different thread?
> 
> Well, it can, since I'm seeing it happen, but is that okay or a bug?

Yes, it can happen. At least for devices like IDE where a request is
started during a vmexit (MMIO or I/O port write), the coroutine will
usually begin its life in the vcpu thread and then move to the main loop
thread.

This is not a problem because the vcpu thread holds the BQL while
running the request coroutine.

> I.e. in a backup-job the following can sporadically trip:
> 
>   unsigned long tid = pthread_self();
>   qemu_get_current_aio_context(); // returns main context
>   qemu_coroutine_yield();
>   qemu_get_current_aio_context(); // still returns main context, but:
>   assert(tid == pthread_self()); // this fails
> 
> It seems to be called from a vCPU thread when it happens. VM uses no
> iothreads.

This must be a guest request that was intercepted by the backup job. I
think it wouldn't happen for the background copy, these requests already
start in the main loop.

Kevin