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

To: "Ian Campbell" <Ian.Campbell@xxxxxxxxxxxxx>, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH] switch to a known good/static GDT beforekexec
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Thu, 02 Jul 2009 12:45:16 +0100
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>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 02.07.09 13:11 >>>
>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).


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