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xen-devel
[Xen-devel] Re: [RFC PATCH 18/33] Subarch support for CPUID instruction
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To: |
Arjan van de Ven <arjan@xxxxxxxxxxxxx> |
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Subject: |
[Xen-devel] Re: [RFC PATCH 18/33] Subarch support for CPUID instruction |
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From: |
Zachary Amsden <zach@xxxxxxxxxx> |
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Date: |
Tue, 18 Jul 2006 04:33:52 -0700 |
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Cc: |
Andrew Morton <akpm@xxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Ian Pratt <ian.pratt@xxxxxxxxxxxxx>, Rusty Russell <rusty@xxxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Andi Kleen <ak@xxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx, Christian Limpach <Christian.Limpach@xxxxxxxxxxxx> |
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Tue, 18 Jul 2006 04:34:19 -0700 |
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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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