Re: [PATCH] arm64: support __int128 with clang

[Mark Rutland]

On Tue, Apr 24, 2018 at 02:35:27PM +0100, Will Deacon wrote:
> On Sun, Apr 15, 2018 at 06:20:11PM +0200, Jason A. Donenfeld wrote:
> > Would you review/merge this when you have a chance?
> 
> No real objection from me, but does the arm64 kernel actually build with
> clang?

I belevie that currently it is not possible to use a released version of
clang to build a working kernel without additional patches (at least
5.0.0, there don't seem to be any 6.0.0 binaries that can target
AArch64). Issues include:

* Released versions of clang don't support the 's' constraint used in
  some hyp code.

* Released versions of clang don't support 'rX' as a register name, as
  is used by some workaround code shared between arm/arm64.

* Additional flags need to be passed for hyp code to function correctly,
  since clang will convert switch statements into jump tables, meaning
  they aren't position-independent.

I tried building a tip-of-tree clang recently (r330441 with llvm
r330440) to see what was solved so far, but this blew up when bulding
the kernel, and I was going to wait for the 7.0.0 release before trying
again.

Thanks,
Mark.