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Re: [Xen-devel] [PATCH, RFC] x86: make the GDT per-CPU

To: "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH, RFC] x86: make the GDT per-CPU
From: "Jan Beulich" <jbeulich@xxxxxxxxxx>
Date: Thu, 11 Sep 2008 13:42:49 +0100
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>>> "Jan Beulich" <jbeulich@xxxxxxxxxx> 11.09.08 14:28 >>>
>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 11.09.08 12:54 >>>
>>Firstly, we don't really need the LDT and TSS GST slots to be always valid.
>>Actually we always initialise the slot immediately before LTR or LLDT. So we
>>could even have per-CPU LDT and TSS initialisation share a single slot.
>>Then, with the extra reserved page, we'd be good for nearly 512 CPUs.
>No, this would break 32-bits at least: The GDT entry for the selector
>loaded into TR must remain a valid, busy TSS descriptor for the whole
>lifetime of the system. So it can't be shared with the LDT. But even for
>64-bits I would fear using the same GDT slot for both LDT and GDT

Actually, there's a second aspect here, too (again for 32-bits): As a
follow-up patch I'm planning to make the double fault TSS per-CPU, too.
This wasn't as simple with the global GDT as it would be with the per-CPU


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