Messages in this thread | | | Subject | Re: [RFC PATCH v1 00/17] ban the use of _PAGE_XXX flags outside platform specific code | From | "Aneesh Kumar K.V" <> | Date | Wed, 5 Sep 2018 19:33:33 +0530 |
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On 09/05/2018 06:06 PM, Christophe Leroy wrote: > Today flags like for instance _PAGE_RW or _PAGE_USER are used through > common parts of code. > Using those directly in common parts of code have proven to lead to > mistakes or misbehaviour, because their use is not always as trivial > as one could think. > > For instance, (flags & _PAGE_USER) == 0 isn't enough to tell > that a page is a kernel page, because some targets are using > _PAGE_PRIVILEDGED and not _PAGE_USER, so the test has to be > (flags & (_PAGE_USER | _PAGE_PRIVILEDGED)) == _PAGE_PRIVILEDGED > This has to (bad) consequences: > > - All targets must define every bit, even the unsupported ones, > leading to a lot of useless #define _PAGE_XXX 0 > - If someone forgets to take into account all possible _PAGE_XXX bits > for the case, we can get unexpected behaviour on some targets. > > This becomes even more complex when we come to using _PAGE_RW. > Testing (flags & _PAGE_RW) is not enough to test whether a page > if writable or not, because: > > - Some targets have _PAGE_RO instead, which has to be unset to tell > a page is writable > - Some targets have _PAGE_R and _PAGE_W, in which case > _PAGE_RW = _PAGE_R | _PAGE_W > - Even knowing whether a page is readable is not always trivial because: > - Some targets requires to check that _PAGE_R is set to ensure page > is readable > - Some targets requires to check that _PAGE_NA is not set > - Some targets requires to check that _PAGE_RO or _PAGE_RW is set > > Etc .... > > In order to work around all those issues and minimise the risks of errors, > this serie aims at removing all use of _PAGE_XXX flags from powerpc code > and always use pte_xxx() and pte_mkxxx() accessors instead. Those accessors > are then defined in target specific parts of the kernel code.
We recently did on book3s 64.
static inline int pte_present(pte_t pte) { /* * A pte is considerent present if _PAGE_PRESENT is set. * We also need to consider the pte present which is marked * invalid during ptep_set_access_flags. Hence we look for _PAGE_INVALID * if we find _PAGE_PRESENT cleared. */ return !!(pte_raw(pte) & cpu_to_be64(_PAGE_PRESENT | _PAGE_INVALID)); }
So I guess with that pte_present conversion we need to be careful.
Do you have a git tree which I can use to double check?
-aneesh
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