Re: [Xen-devel] Re: [PATCH] switch to a known good/static GDT beforekexec

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 02/07/2009 13:38, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>>> I have one more question then -- when need_full_gdt(n), why do we always
>>> unconditionally rewrite the l1es mapping the reserved gdt entries? Shouldn't
>>> that be done on vcpu creation only and perhaps on compat mode switch (i.e.,
>>> isn't it wasteful to do it in the context switch path)?
>> 
>> While that's correct, we're talking about a single memory write here (plus
>> the setup code), which I think is no more wasteful than adding the extra
>> checking that would be needed to see whether we switch between
>> compat and !compat (including the !need_full_gdt(p) case, where we
>> don't know what kind of mapping we currently have).
> 
> Hm yes, fair enough.

Actually no, I'm not sure what extra checking you think you'd need? You'd
create the per-vcpu mapping when a vcpu is created and when it changes
to/from compat mode, and that would be it wouldn't it? You'd only delete
code from __context_switch(); not add different code to it?

 -- Keir



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