On Wed, 12 Feb 2020 15:12:54 +0530 Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote: > >> +/* > >> + * On s390 platform, the lower 12 bits are used to identify given page table > >> + * entry type and for other arch specific requirements. But these bits might > >> + * affect the ability to clear entries with pxx_clear(). So while loading up > >> + * the entries skip all lower 12 bits in order to accommodate s390 platform. > >> + * It does not have affect any other platform. > >> + */ > >> +#define RANDOM_ORVALUE (0xfffffffffffff000UL) > > > > I'd suggest you generate this mask with something like > > GENMASK(BITS_PER_LONG, PAGE_SHIFT). > > IIRC the lower 12 bits constrains on s390 platform might not be really related > to it's PAGE_SHIFT which can be a variable, but instead just a constant number. > But can definitely use GENMASK or it's variants here. > > https://lkml.org/lkml/2019/9/5/862 PAGE_SHIFT would be fine, it is 12 on s390. However, in order to be more precise, we do not really need all 12 bits, only the last 4 bits. So, something like this would work: #define RANDOM_ORVALUE GENMASK(BITS_PER_LONG - 1, 4) The text in the comment could then also be changed from 12 to 4, and be a bit more specific on the fact that the impact on pxx_clear() results from the dynamic page table folding logic on s390: /* * On s390 platform, the lower 4 bits are used to identify given page table * entry type. But these bits might affect the ability to clear entries with * pxx_clear() because of how dynamic page table folding works on s390. So * while loading up the entries do not change the lower 4 bits. * It does not have affect any other platform. */
