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

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