[RFC PATCH 0/2] Add support for a fake, para-virtualised machine

On 12/04/2012 06:30 AM, Will Deacon wrote:

Hi Rob,

[fixing Arnd's address, as I apparently can't spell]

On Mon, Dec 03, 2012 at 09:54:09PM +0000, Rob Herring wrote:

quoted

On 12/03/2012 11:52 AM, Will Deacon wrote:

quoted

When running Linux on a para-virtualised platform (that is, one where
the guest is aware that it is dealing with virtual devices sitting on
things like virtio or xenbus) we require very little in the way of
platform code and piggy-backing on top of an existing platform can
require a lot of device emulation for very little gain.

These two patches introduce mach-virt: a very simple, DT-based machine
which can be used with kvmtool in conjunction with virtio-based devices.
It's not hard to imagine the same machine being targetted by Xen, which
currently emulates a minimal variant of the vexpress platform.

Note that this patch series depends on the timer rework from Mark
Rutland, posted on Friday:

  http://lists.infradead.org/pipermail/linux-arm-kernel/2012-November/135651.html

All feedback welcome. We suspect that most controversy will be around
the name of the thing :)

We've discussed this before at conferences. I don't know that we
concluded this wasn't needed, but it certainly leaned that direction.

I too leaned that direction before I started looking at kvm in detail and,
since then, I've changed my mind when it comes to para-virtualisation.

The reason for this is that there is absolutely no reason to emulate some
components of a real platform and then bolt virtual devices onto it once
you've got enough to get it going. It leads to a right royal mess in
userspace, where you have to write a load of non-reusable emulation code and
it leads to churn in the kernel because you're constantly at odds with
people trying to develop the platform code based on the actual hardware
they have.

With a virtualisation-capable ARMv7 system, all you *need* to boot SMP
Debian is:

	- A v7 CPU with virt extensions
	- vGIC
	- architected timers

*everything* else can be described using virtio devices in the device-tree,
essentially allowing you to generate platforms based on the above and boot
the same kernel on them.

quoted

So what has changed? You're not going to save code space because we're
building multiple platforms together. You'll save some boot time, but a
stripped down dtb with only the minimal peripherals would probably save
nearly as much time. 

It's really got nothing to do with code space or boot speed. What it *is*
about is avoiding the tight coupling with a real platform and suffering as a
result. Yes, you can strip down the DT for a real platform but you'll likely
still have to emulate things like the SP804 in order to boot. That's not to
mention any platform-specific system register interfaces which are required
early on.

We can't even re-use the socfpga code (which is incredibly minimal) without
emulating the dw_apb_timer.

That to me is highlighting where we need to do more work on DT driving
the initialization. The platforms are still aware of what kind of timers
and interrupt controllers are present. They should not be. There's work
in progress for both of those.

Lorenzo's DT MPIDR patches should trim down smp code some. The DT spin
table code could probably be common. I think I could use it on highbank
as well. If we decide the pen code stays, then it should be common
rather than creating yet another copy.

Rob