[Xen-devel] a quick question about _PAGE_GLOBAL flag in para virt mode

[weiming <zephyr.zhao@xxxxxxxxx>]

Hi,

I noticed that in adjust_guest_l1e()  in arch/x86/mm.c, _PAGE_GLOBAL flag is set for guest's non-kernel pages. Meanwhile, _PAGE_GUEST_KERNEL page cannot have the Global bit set. 

What's the purpose of setting Global bit for guest's user pages? AFAIK, when the Global bit is set, it prevents those entries be flushed from TLB. So it makes more sense to set it for guest's kernel pages. 

Below are the codes:
876 #define adjust_guest_l1e(pl1e, d)                                            \

877     do {                                                                     \
878         if ( likely(l1e_get_flags((pl1e)) & _PAGE_PRESENT) &&                \

879              likely(!is_pv_32on64_domain(d)) )                               \

880         {                                                                    \
881             /* _PAGE_GUEST_KERNEL page cannot have the Global bit set. */    \

882             if ( (l1e_get_flags((pl1e)) & (_PAGE_GUEST_KERNEL|_PAGE_GLOBAL)) \

883                  == (_PAGE_GUEST_KERNEL|_PAGE_GLOBAL) )                      \

884                 MEM_LOG("Global bit is set to kernel page %lx",              \

885                         l1e_get_pfn((pl1e)));                                \

886             if ( !(l1e_get_flags((pl1e)) & _PAGE_USER) )                     \

887                 l1e_add_flags((pl1e), (_PAGE_GUEST_KERNEL|_PAGE_USER));      \

888             if ( !(l1e_get_flags((pl1e)) & _PAGE_GUEST_KERNEL) )             \

889                 l1e_add_flags((pl1e), (_PAGE_GLOBAL|_PAGE_USER));            \

890         }                                                                    \
891     } while ( 0 )



Thanks
weiming
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