Support mitigations that are normally done automatically by compilers in assembly

[briansmith]

Rust is adding a -Zharden-sls mitigation for straight-line speculation; see rust-lang/rust#136597.

This raises the question of what to about our external assembly code. Should we expect the assembler to inject these mitigations? If so, which versions of which assemblers will?

Should we modify (ideally upstream) the (Perl)asm source to insert these mitigations? If so, we should coordinate with them.

[briansmith]

Background material:

• https://patchwork.kernel.org/project/xen-devel/patch/fd18939c-cfc7-6de8-07f2-217f810afde1@suse.com/#23683471
• https://www.phoronix.com/news/Arm-Straight-Line-Speculation
• https://lore.kernel.org/lkml/3f61af0eee9b495e8e8c032902d033c5@AcuMS.aculab.com/T/#m891396b53735e58e87265343c7db46f0501057db
• https://www.phoronix.com/news/x86-SLS-Mitigation-5.17

Most of the info about SLS is regarding the initial AArch64 issue but there is also x86-64, at least.