Re: [PATCH] arm: Replace CONFIG_HAS_TLS_REG with HWCAP_TLS and check for it on V6

[Jamie Lokier <jamie@shareable.org>]

Tony Lindgren wrote:
> Also, I wonder if the change __kuser_get_tls is safe?
> 
> +	ldr     r0, [pc, #(16 - 8)]		@ TLS set at 0xffff0ff0?
> +	cmp	r0, #0				@ assume hw TLS if not set
> +	mrceq	p15, 0, r0, c13, c0, 3		@ read TLS register

You cannot assume the TLS value is non-zero, because it's provided by
userspace to use however it wants.  It doesn't even have to be an address.

I'm thinking, why not an alternative() macro like on x86, which is a
very nice way to describe run-time patches of one or a few instructions
which depend on arch feature bits.

Then all that switch_to() logic could be made the size it was before.

An alternative() macro could make a lot of other chip-dependent calls
smaller too, i.e. all those which dispatch through function pointers
at present for cache flushing etc - they could become direct calls, or
an inline instruction or two when possible.

-- Jamie