[PATCH] powerpc: fix hardware PMU exception bug on PowerVM compatibility mode systems

[Leonardo Bras]

Hello Desnes, thanks for the patch.

"Desnes A. Nunes do Rosario" <desnesn at linux.ibm.com> writes:

> PowerVM systems running compatibility mode on a few Power8 revisions are
> still vulnerable to the hardware defect that loses PMU exceptions arriving
> prior to a context switch.
>
> The software fix for this issue is enabled through the CPU_FTR_PMAO_BUG
> cpu_feature bit, nevertheless this bit also needs to be set for PowerVM
> compatibility mode systems.
>
> Fixes: 68f2f0d431d9ea4 ("powerpc: Add a cpu feature CPU_FTR_PMAO_BUG")
> Signed-off-by: Desnes A. Nunes do Rosario <desnesn at linux.ibm.com>
> ---
>  arch/powerpc/kernel/cputable.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/powerpc/kernel/cputable.c b/arch/powerpc/kernel/cputable.c
> index e745abc5457a..5bfef6263dfb 100644
> --- a/arch/powerpc/kernel/cputable.c
> +++ b/arch/powerpc/kernel/cputable.c
> @@ -2198,6 +2198,8 @@ static struct cpu_spec * __init setup_cpu_spec(unsigned long offset,
>  		if (old.oprofile_cpu_type != NULL) {
>  			t->oprofile_cpu_type = old.oprofile_cpu_type;
>  			t->oprofile_type = old.oprofile_type;
> +			if (old.cpu_features & CPU_FTR_PMAO_BUG)
> +				t->cpu_features |= CPU_FTR_PMAO_BUG;

What are your thoughts about doing:
	t->cpu_features |=  old.cpu_features & CPU_FTR_PMAO_BUG;

Also, I would recommend adding a short comment on top of the added
lines explaining why it is needed. 

Best regards,
Leonardo Bras

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