From: Arnd Bergmann <arnd@arndb.de>
To: qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api
Date: Wed, 15 Feb 2012 22:21:35 +0000
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Cc: Alexander Graf <agraf@suse.de>, michael@ellerman.id.au,
        KVM list <kvm@vger.kernel.org>,
        "linux-kernel" <linux-kernel@vger.kernel.org>,
        Eric Northup <digitaleric@google.com>,
        Scott Wood <scottwood@freescale.com>, Avi Kivity <avi@redhat.com>
References: <4F2AB552.2070909@redhat.com> <1328597934.6802.6.camel@concordia> <F1B17875-23E2-4965-BE05-6A3858995D21@suse.de>
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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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