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

[Zachary Amsden <zach@xxxxxxxxxx>]

Arjan van de Ven wrote:
> On Tue, 2006-07-18 at 00:00 -0700, Chris Wright wrote:
>   
> > plain text document attachment (i386-cpuid)
> > Allow subarchitectures to modify the CPUID instruction.  This allows
> > the subarch to provide a limited set of CPUID feature flags during CPU
> > identification.  Add a subarch implementation for Xen that traps to the
> > hypervisor where unsupported feature flags can be hidden from guests.
> >     
> Hi,
>
> I'm wondering if this is entirely the wrong level of abstraction; to me
> it feels the subarch shouldn't override the actual cpuid, but the cpu
> feature flags that linux uses. That's a lot less messy: cpuid has many
> many pieces of information which are near impossible to filter in
> practice, however filtering the USAGE of it is trivial; linux basically
> flattens the cpuid namespace into a simple bitmap of "what the kernel
> can use". That is really what the subarch should filter/fixup, just like
> we do for cpu quirks etc etc.
>   
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.
Zach

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