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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

weiming wrote:
> 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. 
In 64-bit, syscalls must go via Xen, which entails a cr3 reload.  This 
would normally flush the tlb, but since the usermode pages are global, 
they remain in the tlb during the user->xen->kernel->xen->user transition.
   J

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