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

Ira Weiny
2020-07-17T20:53:12Z

On Fri, Jul 17, 2020 at 10:54:42AM +0200, Peter Zijlstra wrote:
> On Fri, Jul 17, 2020 at 12:20:41AM -0700, ira.weiny@intel.com wrote:
> > +/*
> > + * Get a new pkey register value from the user values specified.
> > + *
> > + * Kernel users use the same flags as user space:
> > + * PKEY_DISABLE_ACCESS
> > + * PKEY_DISABLE_WRITE
> > + */
> > +u32 get_new_pkr(u32 old_pkr, int pkey, unsigned long init_val)
> > +{
> > + int pkey_shift = (pkey * PKR_BITS_PER_PKEY);
> > + u32 new_pkr_bits = 0;
> > +
> > + /* Set the bits we need in the register: */
> > + if (init_val & PKEY_DISABLE_ACCESS)
> > + new_pkr_bits |= PKR_AD_BIT;
> > + if (init_val & PKEY_DISABLE_WRITE)
> > + new_pkr_bits |= PKR_WD_BIT;
> > +
> > + /* Shift the bits in to the correct place: */
> > + new_pkr_bits <<= pkey_shift;
> > +
> > + /* Mask off any old bits in place: */
> > + old_pkr &= ~((PKR_AD_BIT | PKR_WD_BIT) << pkey_shift);
> > +
> > + /* Return the old part along with the new part: */
> > + return old_pkr | new_pkr_bits;
> > +}
>
> This is unbelievable junk...
>
> 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.

On the code I think this is fair. I've also updated the calling function to be
a bit cleaner as well.

However, I think the name 'update' is a bit misleading. Here is the new
calling code:

...
pkru = read_pkru();
pkru = update_pkey_reg(pkru, pkey, init_val);
write_pkru(pkru);
...

I think it is odd to have a function called update_pkey_reg() called right
before a write_pkru(). Can we call this update_pkey_value? or just 'val'?
Because write_pkru() actually updates the register.

Thanks for the review,
Ira