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Re: [Xen-devel] which cpuid bits are exposed to pv guest OS and why?

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel (E-mail)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] which cpuid bits are exposed to pv guest OS and why?
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 18 Nov 2009 22:32:44 +0000
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On 18/11/2009 22:01, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> The "other" hypervisor reported these features that
> Xen did not:  vme, pse, mtrr, pge, mca, pse36, lm.
> Xen reported these features that the "other" hypervisor
> did not: ht (only).

None of these are really useful directly to userspace, so it probably
doesn't matter much what gets advertised there. To the guest kernel, most of
the above stuff is paravirtualised or unavailable to it:
 - VME: vm86 cpu feature - entirely unavailable on 64-bit Xen
 - PSE: superpage support - unavailable to pv guests by default
 - MTRR: MTRRs - paravirtualised, so native MSR interface unavailable
 - PGE: global-page pte flag - unavailable to pv guests
 - MCA: machine-check architecture - paravirtualised, native interface
 - PSE36: Like PSE
 - LM: 64-bit support - unavailable to 32-bit guests
 - HT: Hyperthreading/multicore - vCPU probe and bringup is paravirtualised
(so a bit weird we expose this one, but doesn't matter really either way)

 -- Keir

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