Re: [PATCH] arm64/boot: Don't set PMUv3p9 FGT2 bits without PMUv3

[Fuad Tabba]

On Fri, Oct 02, 2026 at 03:51:16PM +0100, Catalin Marinas wrote:
> I couldn't figure out the #8 either. I wonder whether it's more readable
> as (untested):
>
>     sub    x1, x1, #ID_AA64DFR0_EL1_PMUVer_V3P9
>     cmp    x1, #(ID_AA64DFR0_EL1_PMUVer_IMP_DEF - ID_AA64DFR0_EL1_PMUVer_V3P9)
>     b.hs    .Lskip_pmuv3p9_\@    // Skip unless V3P9 <= PMUVer < IMP_DEF

This looks correct to me.

That said, if readability is the goal, I think the plainest is two
compares and two branches. It's one more instruction, but this runs
once at boot:

      cmp     x1, #ID_AA64DFR0_EL1_PMUVer_IMP_DEF
      b.eq    .Lskip_pmuv3p9_\@
      cmp     x1, #ID_AA64DFR0_EL1_PMUVer_V3P9
      b.lt    .Lskip_pmuv3p9_\@

I used ccmp to match __init_el2_debug in the same file, which does the
same NI/IMP_DEF check with a raw #4, as does reset_pmuserenr_el0 in
assembler.h (and hyp-entry.S uses the same pattern for HVC64/HVC32).

Cheers,
/fuad