Detect presence of LPAE when not running an LPAE kernel?

[Ian Campbell]

On Fri, 2013-10-04 at 17:09 +0100, Will Deacon wrote:
> On Fri, Oct 04, 2013 at 08:15:08AM +0100, Ian Campbell wrote:
> > On Thu, 2013-10-03 at 21:07 +0100, Russell King - ARM Linux wrote:
> > > On Thu, Oct 03, 2013 at 08:43:23PM +0100, Ian Campbell wrote:
> > > > Is this possible?
> > > > 
> > > > The Features field of /proc/cpuinfo only appears to reflect the presence
> > > > of LPAE if CONFIG_LPAE is actually on in the running kernel.
> > > > 
> > > > The reason I care is so that distro installers can ship a generic kernel
> > > > but select LPAE for the running system when appropriate.
> > > > 
> > > > On x86 for example I would check /proc/cpuinfo:flags which contains the
> > > > CPU features, regardless of whether they are currently enabled.
> > > > 
> > > > Is there any equivalent on ARM?
> > > 
> > > Perversely, LPAE in there has nothing to do with LPAE being used by the
> > > kernel.  It's more to do with the presence of the double-word load/store
> > > exclusive.
> > 
> > Ah, and I'm running 3.10 whereas this was added to 3.11-rc1.
> 
> Unless my memory is broken, if you see this HWCAP in a mainline kernel it
> has always meant "the CPU you are running on is capable of LPAE". I don't
> remember any dependency on CONFIG_LPAE.

I was just confused by it not appearing in 3.10 and assumed it was a
dependency on CONFIG_LPAE when in reality the hwcap just wasn't added
until 3.11-rc1.

> > Should/do we expose the actual CPU LPAE capability somewhere?
> 
> That's the whole point of the HWCAP.

Russell was saying it actually related to the presence of strd/ldrd, how
tightly is that tied to the presence of LPAE in theory? In practise it
seems closely related...

> Are you sure your 3.10 hasn't got some extra magic?

Yep ;-)

Ian.