Re: [PATCH RFC V2 02/17] x86/fpu: Refactor arch_set_user_pkey_access() for PKS support

Peter Zijlstra
2020-07-20T09:14:17Z

On Fri, Jul 17, 2020 at 03:36:12PM -0700, Dave Hansen wrote:
> On 7/17/20 1:54 AM, Peter Zijlstra wrote:
> > This is unbelievable junk...
>
> Ouch!
>
> This is from the original user pkeys implementation.

The thing I fell over most was new in this patch; the naming of that
function. It doesn't 'get' anything, nor does it allocate anything, so
'new' is out the window too.

> > How about something like:
> >
> > u32 update_pkey_reg(u32 pk_reg, int pkey, unsigned int flags)
> > {
> > int pkey_shift = pkey * PKR_BITS_PER_PKEY;
> >
> > pk_reg &= ~(((1 << PKR_BITS_PER_PKEY) - 1) << pkey_shift);
> >
> > if (flags & PKEY_DISABLE_ACCESS)
> > pk_reg |= PKR_AD_BIT << pkey_shift;
> > if (flags & PKEY_DISABLE_WRITE)
> > pk_reg |= PKR_WD_BIT << pkey_shift;
> >
> > return pk_reg;
> > }
> >
> > Then we at least have a little clue wtf the thing does.. Yes I started
> > with a rename and then got annoyed at the implementation too.
>
> That's fine, if some comments get added.

I'm not sure what you would want commented; the code is trivial.

> It looks correct to me but
> probably compiles down to pretty much the same thing as what was there.
> FWIW, I prefer the explicit masking off of two bit values to implicit
> masking off with a mask generated from PKR_BITS_PER_PKEY. It's
> certainly more compact, but I usually don't fret over the lines of code.

This way you're sure there are no bits missed. Both the shift and mask
use the same value.