From: Dave Hansen <[email protected]> All memory being mapped out to userspace is shared. That means that it is both safe and _expected_ to have the page table Global bit set.
If a PMD is found without Global set, it either a place that could be performing better, or something unexpected is being mapped out to userspace. Both of those are things for which a warning is good. Signed-off-by: Dave Hansen <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Borislav Petkov <[email protected]> Cc: Brian Gerst <[email protected]> Cc: Denys Vlasenko <[email protected]> Cc: H. Peter Anvin <[email protected]> Cc: Josh Poimboeuf <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Rik van Riel <[email protected]> --- b/arch/x86/mm/kpti.c | 3 +++ 1 file changed, 3 insertions(+) diff -puN arch/x86/mm/kpti.c~kaiser-set-global-in-kernel-for-shared arch/x86/mm/kpti.c --- a/arch/x86/mm/kpti.c~kaiser-set-global-in-kernel-for-shared 2017-12-15 09:47:52.884717268 -0800 +++ b/arch/x86/mm/kpti.c 2017-12-15 10:00:39.134715357 -0800 @@ -157,6 +157,9 @@ kpti_clone_pmds(unsigned long start, uns if (WARN_ON(!target_pmd)) return; + /* Only clone PMDs which we *intend* to share: */ + WARN_ON_ONCE(!(pmd_flags(*target_pmd) & _PAGE_GLOBAL)); + /* * Copy the PMD. That is, the kernelmode and usermode * tables will share the last-level page tables of this _
