Detect presence of LPAE when not running an LPAE kernel?

[Ian Campbell]

On Fri, 2013-10-04 at 17:26 +0100, Will Deacon wrote:
> On Fri, Oct 04, 2013 at 05:23:22PM +0100, Ian Campbell wrote:
> > 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:
> > > > 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...
> 
> Take a gander at the code: we're checking the VMSA version in one of the
> MMFR registers. LPAE implies atomic ldrd/strd, so that's what this is all
> about (and why it ended up being called "lpae").

Yes, I see what you mean (check for VMSA >= 5, which is "adds support
for the Long-descriptor translation table format").

You can determine the presence of ldrd/strd from this but that's a bit
of a red herring in the context. I thought Russell was saying the kernel
was checking for a ldrd/strd bit in some feature register and inferring
the lpae flag in /proc/cpuinfo from that, which was why I was concerned.

> In short, this HWCAP does exactly what you want.

Agreed, thanks.

Ian