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

From: Paolo Bonzini
Subject: Re: [PATCH v4 3/4] Introduce the NVMM impl
Date: Wed, 11 Mar 2020 22:44:46 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 11/03/20 22:21, Maxime Villard wrote:
>> 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

How do you ensure that a guest with interrupts off exits promptly?

> , 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.

You can also do what KVM did until a few years ago: swap the signal mask
atomically when you enter the hypervisor (e.g. unmasking SIGUSR1---this
has to be done in the kernel) and when you leave it.  Then in QEMU you
keep SIGUSR1 masked and "eat" it with sigwaitinfo.

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

If you say so.


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