Re: [PATCH v4 18/29] arm64: add POE signal support

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Aug 15, 2024 at 04:09:26PM +0100, Dave P Martin wrote:
> On Thu, Aug 15, 2024 at 02:18:15PM +0100, Joey Gouly wrote:
> > That's a lot of words to say, or ask, do you agree with the approach of only
> > saving POR_EL0 in the signal frame if num_allocated_pkeys() > 1?
> > 
> > Thanks,
> > Joey
> 
> ...So..., given all the above, it is perhaps best to go back to
> dumping POR_EL0 unconditionally after all, unless we have a mechanism
> to determine whether pkeys are in use at all.

Ah, I can see why checking for POR_EL0_INIT is useful. Only checking for
the allocated keys gets confusing with pkey 0.

Not sure what the deal is with pkey 0. Is it considered allocated by
default or unallocatable? If the former, it implies that pkeys are
already in use (hence the additional check for POR_EL0_INIT). In
principle the hardware allows us to use permissions where the pkeys do
not apply but we'd run out of indices and PTE bits to encode them, so I
think by default we should assume that pkey 0 is pre-allocated.

So I agree that it's probably best to save it unconditionally.

-- 
Catalin