Re: [PATCH V2] arm64/cpufeature: Add get_arm64_ftr_reg_nowarn()

Will Deacon
2020-05-27T07:53:19Z

On Wed, May 27, 2020 at 07:56:30AM +0530, Anshuman Khandual wrote:
>
>
> On 05/27/2020 01:16 AM, Will Deacon wrote:
> > On Tue, May 26, 2020 at 04:01:35PM +0100, Catalin Marinas wrote:
> >> On Tue, May 26, 2020 at 07:09:13PM +0530, Anshuman Khandual wrote:
> >>> @@ -632,8 +654,6 @@ static void __init init_cpu_ftr_reg(u32 sys_reg, u64 new)
> >>> const struct arm64_ftr_bits *ftrp;
> >>> struct arm64_ftr_reg *reg = get_arm64_ftr_reg(sys_reg);
> >>>
> >>> - BUG_ON(!reg);
> >>> -
> >>> for (ftrp = reg->ftr_bits; ftrp->width; ftrp++) {
> >>> u64 ftr_mask = arm64_ftr_mask(ftrp);
> >>> s64 ftr_new = arm64_ftr_value(ftrp, new);
> >>> @@ -762,7 +782,6 @@ static int check_update_ftr_reg(u32 sys_id, int cpu, u64 val, u64 boot)
> >>> {
> >>> struct arm64_ftr_reg *regp = get_arm64_ftr_reg(sys_id);
> >>>
> >>> - BUG_ON(!regp);
> >>> update_cpu_ftr_reg(regp, val);
> >>> if ((boot & regp->strict_mask) == (val & regp->strict_mask))
> >>> return 0;
> >>> @@ -776,9 +795,6 @@ static void relax_cpu_ftr_reg(u32 sys_id, int field)
> >>> const struct arm64_ftr_bits *ftrp;
> >>> struct arm64_ftr_reg *regp = get_arm64_ftr_reg(sys_id);
> >>>
> >>> - if (WARN_ON(!regp))
> >>> - return;
> >>
> >> I think Will wanted an early return in all these functions not just
> >> removing the BUG_ON(). I'll let him clarify.
> >
> > Yes, the callers need to check the pointer and return early.
>
> Sure, will do. But for check_update_ftr_reg(), a feature register search
> failure should be treated as a success (0) or a failure (1). What should
> it return ? Seems bit tricky, as there are good reasons to go either way.

We're unable to check it so return 0, otherwise we'll randomly taint the
kernel and print a weird message.

Will