Re: [PATCH v4 3/4] Introduce the NVMM impl

[Maxime Villard]

Le 11/03/2020 à 21:42, Paolo Bonzini a écrit :
> On 11/03/20 21:14, Maxime Villard wrote:
>>> The problem is that qcpu->stop is checked _before_ entering the
>>> hypervisor and not after, so there is a small race window.
>> Ok. I don't understand what's supposed to be the race here. If we get an
>> IPI between the check and the call to nvmm_vcpu_run() then we'll just do
>> one run and stop in the next iteration, because the IPI will have set
>> qcpu->stop. Is this extra iteration undesired?
> 
> Yes, you don't know how long that run would take.  I don't know about
> NVMM but for KVM it may even never leave if the guest is in HLT state.

Ok, I see, thanks.

In NVMM the runs are short, the syscalls are fast, and pending signals
cause returns to userland. Therefore, in practice, it's not a big problem,
because (1) the window is small and (2) if we have a miss it's not going
to take long to come back to Qemu.

I see a quick kernel change I can make to reduce 95% of the window
already in the current state. The remaining 5% will need a new
nvmm_vcpu_kick() function.

For now this issue is unimportant and no Qemu change is required.

Kamil, please also drop the XXX in
    /* XXX Needed, otherwise infinite loop. */
It's not a bug.

Thanks,
Maxime