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Re: [Qemu-devel] [RFC PATCH] Implement qemu_thread_yield for posix, use
Re: [Qemu-devel] [RFC PATCH] Implement qemu_thread_yield for posix, use it in mttcg to handle EXCP_YIELD
Wed, 22 Jan 2020 08:01:18 -1000
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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
>> 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():
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.