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

From: Zachary Amsden
Date: Tue Jul 18 2006 - 07:32:13 EST

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.


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.

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