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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: |
Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> |
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Date: |
Tue, 18 Jul 2006 11:26:19 +0100 |
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Cc: |
Andrew Morton <akpm@xxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Ian Pratt <ian.pratt@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Chris Wright <chrisw@xxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx |
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Tue, 18 Jul 2006 03:26:49 -0700 |
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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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