Re: [Xen-devel] [PATCH] Optimizing x86-64 xenlinux using global pages for user mode (take 2)

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 30/8/06 7:43 am, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx> wrote:

> The previous patch had a problem with the builder, which constructs the
> initial page tables with the USER bit on. And Xen couldn't distinguish
> kernel or use pages. It was partially checked avoiding the bug, and it's
> finished with this patch.

I'd like to avoid awareness of the extra PAGE_GUEST_KERNEL bit in guests and
tools as much as possible (in fact, entirely would be good).

There's no need for all the PAGE_KERNEL macros in Linux to include the new
flag. The absence of PAGE_USER will suffice to allow Xen to distinguish
kernel entries from user entries. All that is necessary is for Xen to add
the flag when it adds PAGE_USER to a kernel entry, so that kernel entries
remain disambiguated from user entries.

Also, you should use one of the high-order available bits (one of bits
52-60). Other OSes use the low-order available bits for their own purposes
-- we may as well avoid conflicting with them.

 -- Keir



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