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Subject: Re: [PATCH v2] kvm: Replace vcpu->swait with rcuwait
To:     Davidlohr Bueso <dave@stgolabs.net>
Cc:     Marc Zyngier <maz@kernel.org>, tglx@linutronix.de,
        kvm@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>,
        peterz@infradead.org, torvalds@linux-foundation.org,
        bigeasy@linutronix.de, linux-kernel@vger.kernel.org,
        rostedt@goodmis.org, linux-mips@vger.kernel.org,
        Paul Mackerras <paulus@ozlabs.org>, joel@joelfernandes.org,
        will@kernel.org, kvmarm@lists.cs.columbia.edu
References: <20200324044453.15733-1-dave@stgolabs.net>
 <20200324044453.15733-4-dave@stgolabs.net>
 <20200420164132.tjzk5ebx35m66yce@linux-p48b>
 <418acdb5001a9ae836095b7187338085@misterjones.org>
 <20200420205641.6sgsllj6pmsnwrvp@linux-p48b>
 <f7cc83fe-3e91-0057-9af2-26c201456689@redhat.com>
 <20200420215014.sarodevmhphnkkn7@linux-p48b>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <02e1b00d-a8ea-a947-bbe6-0b1380aa7ec4@redhat.com>
Date:   Tue, 21 Apr 2020 11:43:39 +0200
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On 20/04/20 23:50, Davidlohr Bueso wrote:
> On Mon, 20 Apr 2020, Paolo Bonzini wrote:
> 
>> On 20/04/20 22:56, Davidlohr Bueso wrote:
>>> On Mon, 20 Apr 2020, Marc Zyngier wrote:
>>>
>>>> This looks like a change in the semantics of the tracepoint. Before
>>>> this
>>>> change, 'waited' would have been true if the vcpu waited at all. Here,
>>>> you'd
>>>> have false if it has been interrupted by a signal, even if the vcpu
>>>> has waited
>>>> for a period of time.
>>>
>>> Hmm but sleeps are now uninterruptible as we're using TASK_IDLE.
>>
>> Hold on, does that mean that you can't anymore send a signal in order to
>> kick a thread out of KVM_RUN?? Or am I just misunderstanding?
> 
> Considering that the return value of the interruptible wait is not
> checked, I would not think this breaks KVM_RUN.

What return value?  kvm_vcpu_check_block checks signal_pending, so you
could have a case where the signal is injected but you're not woken up.

Admittedly I am not familiar with how TASK_* work under the hood, but it
does seem to be like that.

Paolo