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SubjectRe: [RFC PATCH 18/33] Subarch support for CPUID instruction
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On 18 Jul 2006, at 11:14, Arjan van de Ven wrote:

>> 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.
>
> 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.

Maybe we should have that *as well*, but it makes sense to allow the
hypervisor to apply a filter too. For example, whether it supports PSE,
FXSAVE/FXRSTOR, etc. These are things the 'platform' is telling the OS
-- not something the OS can filter for itself. To trap on CPUID
invocations requires the guest to use a special code sequence for
CPUID, since the instruction will never normally fault. Hence moving to
mach-* to hide this detail.

-- Keir

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