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

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Date:	Thu, 16 Feb 2012 21:28:14 +0200
From:	Avi Kivity <avi@...hat.com>
To:	michael@...erman.id.au
CC:	Arnd Bergmann <arnd@...db.de>, qemu-devel@...gnu.org,
	Alexander Graf <agraf@...e.de>, KVM list <kvm@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Eric Northup <digitaleric@...gle.com>,
	Scott Wood <scottwood@...escale.com>
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api

On 02/16/2012 03:04 AM, Michael Ellerman wrote:
> > 
> > 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.
>
> Yeah maybe. That distinction is at least in part just historical.
>
> The first problem I see with using a syscall is that you don't need one
> syscall for KVM, you need ~90. OK so you wouldn't do that, you'd use a
> multiplexed syscall like epoll_ctl() - or probably several
> (vm/vcpu/etc).

No.  Many of our ioctls are for state save/restore - we reduce that to
two.  Many others are due to the with/without irqchip support - we slash
that as well.  The device assignment stuff is relegated to vfio.

I still have to draw up a concrete proposal, but I think we'll end up
with 10-15.

>
> Secondly you still need a handle/context for those syscalls, and I think
> the most sane thing to use for that is an fd.

The context is the process (for vm-wide calls) and thread (for vcpu
local calls).

>
> At that point you've basically reinvented ioctl :)
>
> I also think it is an advantage that you have a node in /dev for
> permissions. I know other "core kernel" interfaces don't use a /dev
> node, but arguably that is their loss.

Have to agree with that.  Theoretically we don't need permissions for
/dev/kvm, but in practice we do.


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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