[Xen-devel] Re: [RFC PATCH 18/33] Subarch support for CPUID instruction

[Chris Wright <chrisw@xxxxxxxxxxxx>]

* David Miller (davem@xxxxxxxxxxxxx) wrote:
> From: Zachary Amsden <zach@xxxxxxxxxx>
> Date: Tue, 18 Jul 2006 04:33:52 -0700
> 
> > You really need a CPUID hook.  The instruction is non-virtualizable, and 
> > anything claiming to be a hypervisor really has to support masking and 
> > flattening the cpuid namespace for the instruction itself.  It is used 
> > in assembler code and very early in boot.  The alternative is injecting 
> > a bunch of Xen-specific code to filter feature bits into the i386 layer, 
> > which is both bad for Linux and bad for Xen - and was quite ugly in the 
> > last set of Xen patches.
> 
> Userspace will still see the full set of cpuid bits, since
> it can still execute cpuid unimpeded, is this ok?

Yup, it's for the kernel (things like systenter vs int 80 in vdso).

thanks,
-chris

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