Re: [PATCH v1 1/3] arm64: add a macro for SError synchronization

[Mark Rutland]

On Wed, Nov 01, 2017 at 08:54:44PM +0800, gengdongjiu wrote:
> On 2017/11/1 19:24, Robin Murphy wrote:
> >> +	esb
> >> +alternative_else_nop_endif
> >> +1:
> >> +	.endm
> > Having a branch in here is pretty horrible, and furthermore using label
> > number 1 has a pretty high chance of subtly breaking code where this
> > macro is inserted.
> > 
> > Can we not somehow nest or combine the alternative conditions here?
> 
> I found it will report error if combine the alternative conditions here.
> 
> For example:
> 
> +	.macro	error_synchronize
> +alternative_if ARM64_HAS_IESB
> +alternative_if ARM64_HAS_RAS_EXTN
> +	esb
> +alternative_else_nop_endif
> +alternative_else_nop_endif
> +	.endm
> 
> And even using b.eq/cbz instruction in the alternative instruction in arch/arm64/kernel/entry.S,
> it will report Error.

Alternatives cannot be nested. You need to define a cap like:

  ARM64_HAS_RAS_NOT_IESB

... which is set when ARM64_HAS_RAS_EXTN && !ARM64_HAS_IESB.

Then you can do:

alternative_if ARM64_HAS_RAS_NOT_IESB
	esb
alternative_else_nop_endif

See ARM64_ALT_PAN_NOT_UAO for an example.

That said, as Robin points out we may not even need the alternative.

Thanks,
Mark.