Re: [Xen-devel] CONFIG_XEN_COMPAT_030002 broken?

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 13/11/06 17:08, "Gerd Hoffmann" <kraxel@xxxxxxx> wrote:

>> Yes, and to allow fewer TLB entries to be flushed when switching between
>> guest kernel and guest user. That optimisation is foiled if PAGE_USER is set
>> everywhere.
> 
> Ok, so the extra cost to decide that at runtime (if
> CONFIG_XEN_COMPAT_030002 is set) probably is outweighed by the tlb flush
> optimization ...

Definitely!

The only potential problem is I don't know whether any code depends on those
definitions being compile-time constant. If not, it should be a
straightforward patch.

By the way, the test of whether to poke in PAGE_USER can be done by looking
at one of the initial mappings provided by the domain builder. If one of
those ptes contains PAGE_USER, you know you need to use PAGE_USER for all
kernel mappings.

 -- Keir


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