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

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Message-Id: <F1B17875-23E2-4965-BE05-6A3858995D21@suse.de>
Date:	Tue, 7 Feb 2012 11:04:44 +0100
From:	Alexander Graf <agraf@...e.de>
To:	michael@...erman.id.au
Cc:	Scott Wood <scottwood@...escale.com>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Eric Northup <digitaleric@...gle.com>,
	Avi Kivity <avi@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	qemu-devel <qemu-devel@...gnu.org>
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api


On 07.02.2012, at 07:58, Michael Ellerman wrote:

> On Mon, 2012-02-06 at 13:46 -0600, Scott Wood wrote:
>> On 02/03/2012 04:52 PM, Anthony Liguori wrote:
>>> On 02/03/2012 12:07 PM, Eric Northup wrote:
>>>> On Thu, Feb 2, 2012 at 8:09 AM, Avi Kivity<avi@...hat.com>  wrote:
>>>> [...]
>>>>> 
>>>>> Moving to syscalls avoids these problems, but introduces new ones:
>>>>> 
>>>>> - adding new syscalls is generally frowned upon, and kvm will need
>>>>> several
>>>>> - syscalls into modules are harder and rarer than into core kernel code
>>>>> - will need to add a vcpu pointer to task_struct, and a kvm pointer to
>>>>> mm_struct
>>>> - Lost a good place to put access control (permissions on /dev/kvm)
>>>> for which user-mode processes can use KVM.
>>>> 
>>>> How would the ability to use sys_kvm_* be regulated?
>>> 
>>> Why should it be regulated?
>>> 
>>> It's not a finite or privileged resource.
>> 
>> 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.


Alex

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