Re: [PATCH 07/16] x86/mm: Move two more functions from pgtable_64.h to pgtable.h

[Joerg Roedel]

On Tue, Jan 16, 2018 at 10:03:09AM -0800, Dave Hansen wrote:
> On 01/16/2018 08:36 AM, Joerg Roedel wrote:
> > +	return (((ptr & ~PAGE_MASK) / sizeof(pgd_t)) < KERNEL_PGD_BOUNDARY);
> > +}
> 
> One of the reasons to implement it the other way:
> 
> -	return (ptr & ~PAGE_MASK) < (PAGE_SIZE / 2);
> 
> is that the compiler can do this all quickly.  KERNEL_PGD_BOUNDARY
> depends on PAGE_OFFSET which depends on a variable.  IOW, the compiler
> can't do it.
> 
> How much worse is the code that this generates?

I havn't looked at the actual code this generates, but the
(PAGE_SIZE / 2) comparison doesn't work on 32 bit where the address
space is not always evenly split. I'll look into a better way to check
this.

Thanks,

	Joerg