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

[Fuad Tabba]

On Fri, Oct 02, 2026 at 02:29:26PM +0100, Will Deacon wrote:
> Are you sure #8 is the correct immediate for the ccmp? My reading of the
> pseudocode is that it should be #9, but this instruction has always confused
> me and I hate the fact that it doesn't take another condition code mnemonic
> instead of a raw immediate value.

My read is that #8 is right. From the CCMP (immediate) pseudocode in the
Arm ARM (DDI 0487 M.d, C6.2.81), flags start out as the nzcv immediate
and are only overwritten by the compare when the condition holds:

      var flags : bits(4) = nzcv;
      ...
      if ConditionHolds(condition) then
          ...
          (-, flags) = AddWithCarry{datasize}(operand1, NOT operand2, '1');
      end;
      PSTATE.[N,Z,C,V] = flags;

So when PMUVer == IMP_DEF the ne fails and NZCV = 0b1000, i.e. N=1, V=0.
LT is N != V (Table C1-1), so b.lt is taken and we skip. #9 would give
N=1, V=1, so we'd fall through and set the bits on an IMP_DEF PMU.

Agreed on the raw immediate, it's horrible.

Cheers,
/fuad