Re: [PATCH 1/2] arm: Replace CONFIG_HAS_TLS_REG with HWCAP_TLS and check for it on V6

[Jamie Lokier <jamie@shareable.org>]

Tony Lindgren wrote:
> * Jamie Lokier <jamie@shareable.org> [100622 19:54]:
> > Tony Lindgren wrote:
> > >  __kuser_get_tls:				@ 0xffff0fe0
> > > -
> > > -#if !defined(CONFIG_HAS_TLS_REG) && !defined(CONFIG_TLS_REG_EMUL)
> > > -	ldr	r0, [pc, #(16 - 8)]		@ TLS stored at 0xffff0ff0
> > > -#else
> > > -	mrc	p15, 0, r0, c13, c0, 3		@ read TLS register
> > > -#endif
> > > +	ldr     r0, [pc, #(20 - 8)]		@ software TLS set in 0xffff0ff4?
> > > +	cmp	r0, #0				@ hardware TLS if flag not set
> > > +	mrceq	p15, 0, r0, c13, c0, 3		@ read hardware TLS register
> > > +	ldrne	r0, [pc, #(12 - 8)]		@ software TLS val at 0xffff0ff8
> > >  	usr_ret	lr
> > > -
> > > -	.rep	5
> > > -	.word	0			@ pad up to __kuser_helper_version
> > > -	.endr
> > > +	.word	0				@ non-zero for software TLS
> > > +	.word	0				@ software TLS value
> > 
> > It'd be nice not to waste instructions checking for HWCAP_TLS on archs
> > which definitely don't have it.  I guess it doesn't matter elsewhere;
> > I'd expect this to be a warm path for some programs making extensive
> > use of TLS (I haven't measured though).
> 
> OK, but let's try to figure out a way that does not add more ifdef else
> code as that makes it harder to build support for multiple ARM cores.
>  
> > As it's only a single instruction, and the code is in a writable page
> > already (copied at init), how about just patching the instruction
> > when ELF_HWCAP is set?
> 
> Yeah that can be done for __kuser_get_tls if it's always writable.
> But __switch_to is trickier because of the CONFIG_MMU ifdefs there.

__kuser_get_tls must be writable in kernels where !HAS_TLS_REG is
supported, because the TLS value is written to the same page.

I was thinking of changing *only* __kuser_get_tls, by the way.  Out of
all the different places, that's the only one I'd expect to be a hot
path in some TLS-using programs.

> What if we have optional __switch_to and __kuser_get_tls implementations
> in the mm/proc-*.S files that get copied over the current locations
> if implemented?

As __kuser_get_tls varies by only ones instruction, I don't think
there's any point doing anything other than a single word write, at
the point where the HWCAP is set, with its initial value being the
!HAS_TLS_REG instruction.

For other things like __switch_to and maybe cache maintenance calls,
dmb() etc, I'd suggest first doing a generic asm mechanism like x86's
"alternatives", keeping in mind that one of the alternatives has to be
XIP friendly.

-- Jamie