linux-kernel - Re: [PATCH 4/4] KVM: use signals to abort enter

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Date:   Tue, 25 Oct 2022 12:05:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org
Cc:     Jonathan Corbet <corbet@....net>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] KVM: use signals to abort enter_guest/blocking and
 retry

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