linux-kernel - Re: [PATCH] kvm: don't take vcpu mutex for obviously invalid vcpu ioctls

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Message-ID: <20140922200812.GA9217@amt.cnet>
Date:	Mon, 22 Sep 2014 17:08:12 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	David Matlack <dmatlack@...gle.com>
Cc:	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: don't take vcpu mutex for obviously invalid vcpu
 ioctls

On Fri, Sep 19, 2014 at 04:03:25PM -0700, David Matlack wrote:
> vcpu ioctls can hang the calling thread if issued while a vcpu is
> running. 

There is a mutex per-vcpu, so thats expected, OK...

> If we know ioctl is going to be rejected as invalid anyway,
> we can fail before trying to take the vcpu mutex.

Consider a valid ioctl that takes the vcpu mutex. If you need immediate
access for that valid ioctl, it is necessary to interrupt thread
which KVM_RUN ioctl executes. 

So knowledge of whether KVM_RUN is being executed is expected in
userspace (either
that or ask the KVM_RUN thread to run the ioctl for you, as qemu does).

Can't see why having different behaviour for valid/invalid ioctls
is a good thing.

> This patch does not change functionality, it just makes invalid ioctls
> fail faster.

Should not be executing vcpu ioctls without interrupt KVM_RUN in the
first place.

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