Tony Lindgren wrote: > * Jamie Lokier <ja...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
