Re: [PATCH v4] ELF: AT_PAGE_SHIFT_MASK -- supply userspace with available page shifts

Alexey Dobriyan
2024-02-13T18:51:08Z

On Fri, Feb 09, 2024 at 04:41:36PM -0800, Kees Cook wrote:
> On Fri, Feb 09, 2024 at 03:30:37PM +0300, Alexey Dobriyan wrote:
> > On Mon, Feb 05, 2024 at 04:48:08AM -0800, Kees Cook wrote:
> > > On Mon, Feb 05, 2024 at 12:51:43PM +0300, Alexey Dobriyan wrote:
> > > > +#define ARCH_AT_PAGE_SHIFT_MASK \
> > > > + do { \
> > > > + u32 val = 1 << 12; \
> > > > + if (boot_cpu_has(X86_FEATURE_PSE)) { \
> > > > + val |= 1 << 21; \
> > > > + } \
> > > > + if (boot_cpu_has(X86_FEATURE_GBPAGES)) { \
> > > > + val |= 1 << 30; \
> > > > + } \
> > >
> > > Can we use something besides literal "12", "21", and "30" values here?
> >
> > Ehh, no, why? Inside x86_64 the page shifts are very specific numbers,
> > they won't change.
>
> Well, it's nicer to have meaningful words to describe these things.

Not really. Inside specific arch page shifts are fixed, so using names
is just more macros one need to remember.

If I were to invent names (which I wouldn't), the best names are

PAGE_SHIFT
PAGE_SHIFT2
PAGE_SHIFT3
...

with PAGE_SHIFT2, PAGE_SHIFT3 being optional macros if arch doesn't support
multiple page sizes.

> In fact, PAGE_SHIFT already exists for 12, and HPAGE_SHIFT already exists
> for 21. Please use those, and add another, perhaps GBPAGE_SHIFT, for 30.

HPAGE_SHIFT is bad name, H doesn't describe anything unless arch is
known. Hugepages is marketing name. If GBPAGE_SHIFT is good name,
then HPAGE_SHIFT is bad name, it should've been MBPAGE_SHIFT, which
wrong because it is 2 MiB not 1 MiB.

BTW parisc has REAL_HPAGE_SHIFT !