[PATCH v8 27/28] arm64: mte: Kconfig entry

[Catalin Marinas]

On Mon, Aug 24, 2020 at 11:44:50AM -0700, Randy Dunlap wrote:
> On 8/24/20 11:27 AM, Catalin Marinas wrote:
> > index 6d232837cbee..10cf81d70657 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -1664,6 +1664,37 @@ config ARCH_RANDOM
> >  	  provides a high bandwidth, cryptographically secure
> >  	  hardware random number generator.
> >  
> > +config ARM64_AS_HAS_MTE
> > +	# Binutils gained initial support for MTE in 2.32.0. However, a
> > +	# late architecture addition (LDGM/STGM) is only supported in
> > +	# the newer 2.32.x and 2.33 versions.
> > +	def_bool $(as-instr,.arch armv8.5-a+memtag\nstgm xzr$(comma)[x0])
> 
> Would you mind translating that for me?
> Yes, I read the v7 Notes, but that only helped a little bit.

The initial MTE support in binutils 2.32.0, testable above with ".arch
armv8.5-a+memtag", was incomplete. Historically, I think it was based on
a beta version of the architecture but before the final architecture
release (ARMv8.5), MTE gained a couple of new instructions: STGM/LDGM.

Since there are binutils versions out there which don't understand STGM
even though they claim to support .arch armv8.5-a+memtag, it's better
for the above check to include the STGM instruction.

I'll see if I can make the comment above clearer.

> > +	bool "Memory Tagging Extension support"
> > +	default y
> > +	depends on ARM64_AS_HAS_MTE && ARM64_TAGGED_ADDR_ABI
> > +	select ARCH_USES_HIGH_VMA_FLAGS
> > +	help
> > +	  Memory Tagging (part of the ARMv8.5 Extensions) provides
> > +	  architectural support for run-time, always-on detection of
> 
> 	                            runtime,
> as is used below.

Thanks.

-- 
Catalin