Re: [PATCH v2 01/17] arm64: mm: don't use x18 in idmap_kpti_install_ng_mappings

Mark Rutland
2019-10-25T09:24:34Z

On Thu, Oct 24, 2019 at 03:51:16PM -0700, samitolvanen@google.com wrote:
> idmap_kpti_install_ng_mappings uses x18 as a temporary register, which
> will result in a conflict when x18 is reserved. Use x16 and x17 instead
> where needed.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

AFAICT the new register assignment is sound, so FWIW:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

I was going to suggest adding menmonics for the remamining raw register
names, but after having a go locally I think it's cleaner as-is given
the registers are used in different widths for multiple purposes.

Thanks,
Mark.

> ---
> arch/arm64/mm/proc.S | 63 ++++++++++++++++++++++----------------------
> 1 file changed, 32 insertions(+), 31 deletions(-)
>
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index a1e0592d1fbc..fdabf40a83c8 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -250,15 +250,15 @@ ENTRY(idmap_kpti_install_ng_mappings)
> /* We're the boot CPU. Wait for the others to catch up */
> sevl
> 1: wfe
> - ldaxr w18, [flag_ptr]
> - eor w18, w18, num_cpus
> - cbnz w18, 1b
> + ldaxr w17, [flag_ptr]
> + eor w17, w17, num_cpus
> + cbnz w17, 1b
>
> /* We need to walk swapper, so turn off the MMU. */
> pre_disable_mmu_workaround
> - mrs x18, sctlr_el1
> - bic x18, x18, #SCTLR_ELx_M
> - msr sctlr_el1, x18
> + mrs x17, sctlr_el1
> + bic x17, x17, #SCTLR_ELx_M
> + msr sctlr_el1, x17
> isb
>
> /* Everybody is enjoying the idmap, so we can rewrite swapper. */
> @@ -281,9 +281,9 @@ skip_pgd:
> isb
>
> /* We're done: fire up the MMU again */
> - mrs x18, sctlr_el1
> - orr x18, x18, #SCTLR_ELx_M
> - msr sctlr_el1, x18
> + mrs x17, sctlr_el1
> + orr x17, x17, #SCTLR_ELx_M
> + msr sctlr_el1, x17
> isb
>
> /*
> @@ -353,46 +353,47 @@ skip_pte:
> b.ne do_pte
> b next_pmd
>
> + .unreq cpu
> + .unreq num_cpus
> + .unreq swapper_pa
> + .unreq cur_pgdp
> + .unreq end_pgdp
> + .unreq pgd
> + .unreq cur_pudp
> + .unreq end_pudp
> + .unreq pud
> + .unreq cur_pmdp
> + .unreq end_pmdp
> + .unreq pmd
> + .unreq cur_ptep
> + .unreq end_ptep
> + .unreq pte
> +
> /* Secondary CPUs end up here */
> __idmap_kpti_secondary:
> /* Uninstall swapper before surgery begins */
> - __idmap_cpu_set_reserved_ttbr1 x18, x17
> + __idmap_cpu_set_reserved_ttbr1 x16, x17
>
> /* Increment the flag to let the boot CPU we're ready */
> -1: ldxr w18, [flag_ptr]
> - add w18, w18, #1
> - stxr w17, w18, [flag_ptr]
> +1: ldxr w16, [flag_ptr]
> + add w16, w16, #1
> + stxr w17, w16, [flag_ptr]
> cbnz w17, 1b
>
> /* Wait for the boot CPU to finish messing around with swapper */
> sevl
> 1: wfe
> - ldxr w18, [flag_ptr]
> - cbnz w18, 1b
> + ldxr w16, [flag_ptr]
> + cbnz w16, 1b
>
> /* All done, act like nothing happened */
> - offset_ttbr1 swapper_ttb, x18
> + offset_ttbr1 swapper_ttb, x16
> msr ttbr1_el1, swapper_ttb
> isb
> ret
>
> - .unreq cpu
> - .unreq num_cpus
> - .unreq swapper_pa
> .unreq swapper_ttb
> .unreq flag_ptr
> - .unreq cur_pgdp
> - .unreq end_pgdp
> - .unreq pgd
> - .unreq cur_pudp
> - .unreq end_pudp
> - .unreq pud
> - .unreq cur_pmdp
> - .unreq end_pmdp
> - .unreq pmd
> - .unreq cur_ptep
> - .unreq end_ptep
> - .unreq pte
> ENDPROC(idmap_kpti_install_ng_mappings)
> .popsection
> #endif
> --
> 2.24.0.rc0.303.g954a862665-goog
>