On 02/12/2020 11:25 PM, Gerald Schaefer wrote: > 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. > */ Sure, will update accordingly.
