linux-kernel - Re: [Qemu-devel] [RFC] Next gen kvm api

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Date:	Wed, 15 Feb 2012 22:21:35 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	qemu-devel@...gnu.org
Cc:	Alexander Graf <agraf@...e.de>, michael@...erman.id.au,
	KVM list <kvm@...r.kernel.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	Eric Northup <digitaleric@...gle.com>,
	Scott Wood <scottwood@...escale.com>,
	Avi Kivity <avi@...hat.com>
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api

On Tuesday 07 February 2012, Alexander Graf wrote:
> On 07.02.2012, at 07:58, Michael Ellerman wrote:
> 
> > On Mon, 2012-02-06 at 13:46 -0600, Scott Wood wrote:
> >> You're exposing a large, complex kernel subsystem that does very
> >> low-level things with the hardware.  It's a potential source of exploits
> >> (from bugs in KVM or in hardware).  I can see people wanting to be
> >> selective with access because of that.
> > 
> > Exactly.
> > 
> > In a perfect world I'd agree with Anthony, but in reality I think
> > sysadmins are quite happy that they can prevent some users from using
> > KVM.
> > 
> > You could presumably achieve something similar with capabilities or
> > whatever, but a node in /dev is much simpler.
> 
> Well, you could still keep the /dev/kvm node and then have syscalls operate on the fd.
> 
> But again, I don't see the problem with the ioctl interface. It's nice, extensible and works great for us.
> 

ioctl is good for hardware devices and stuff that you want to enumerate
and/or control permissions on. For something like KVM that is really a
core kernel service, a syscall makes much more sense.

I would certainly never mix the two concepts: If you use a chardev to get
a file descriptor, use ioctl to do operations on it, and if you use a 
syscall to get the file descriptor then use other syscalls to do operations
on it.

I don't really have a good recommendation whether or not to change from an
ioctl based interface to syscall for KVM now. On the one hand I believe it
would be significantly cleaner, on the other hand we cannot remove the
chardev interface any more since there are many existing users.

	Arnd
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