Re: [PATCH 4/4] KVM: use signals to abort enter_guest/blocking and retry

Paolo Bonzini
2022-10-25T10:13:28Z

On 10/24/22 09:43, Emanuele Giuseppe Esposito wrote:
>> Since the userspace should anyway avoid going into this effectively-busy
>> wait, what about clearing the request after the first exit?  The
>> cancellation ioctl can be kept for vCPUs that are never entered after
>> KVM_KICK_ALL_RUNNING_VCPUS.  Alternatively, kvm_clear_all_cpus_request
>> could be done right before up_write().
>
> Clearing makes sense, but should we "trust" the userspace not to go into
> busy wait?

I think so, there are many other ways for userspace to screw up.

> What's the typical "contract" between KVM and the userspace? Meaning,
> should we cover the basic usage mistakes like forgetting to busy wait on
> KVM_RUN?

Being able to remove the second ioctl if you do (sort-of pseudocode
based on this v1)

kvm_make_all_cpus_request(kvm, KVM_REQ_USERSPACE_KICK);
down_write(&kvm->memory_transaction);
up_write(&kvm->memory_transaction);
kvm_clear_all_cpus_request(kvm, KVM_REQ_USERSPACE_KICK);

would be worth it, I think.

Paolo