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

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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