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

[Will Deacon <will@kernel.org>]

On Fri, Oct 02, 2026 at 02:54:47PM +0100, Fuad Tabba wrote:
> 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.

Aha, that table is pretty helpful, thanks.

I think you're right -- I missed the inversion at the end of
ConditionHolds().

I'll pick this up next week.

Will