Re: [Qemu-devel] [RFC PATCH] Implement qemu_thread_yield for posix, use it in mttcg to handle EXCP_YIELD

[Richard Henderson]

On 1/21/20 5:26 PM, David Gibson wrote:
> However, a more common use of qemu is the "pseries" machine type,
> which emulates only a guest (in the cpu architectural sense) with qemu
> taking the place of the hypervisor as well as emulating the cpus.  In
> that case the H_CONFER hypercall goes to qemu.
> 
>> If you are running QEMU as a KVM monitor this is still outside of it's
>> scope as all the scheduling shenanigans are dealt with inside the
>> kernel.
>>
>> From QEMU's TCG point of view we want to concern ourselves with what the
>> real hardware would do - which I think in this case is drop to the
>> hypervisor and let it sort it out.
> 
> Right, but with the "pseries" machine type qemu *is* the hypervisor.

In which case this behaviour doesn't seem implausible.

I will note that "pthread_yield" isn't standardized; "sched_yield" is the one
in POSIX.  Though that says nothing about how that syscall might affect a
hypothetical many-to-one pthread implementation.  You could, I suppose, have a
configure test for pthread_yield.

Also, the win32 implementation would be SwitchToThread():

https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-switchtothread

It looks like one need do nothing for the single-threaded implementation,
qemu_tcg_rr_cpu_thread_fn, as any return to the main loop will select the next
round-robin cpu.  But a note to say that's been tested would be nice.


r~