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xen-devel
[Xen-devel] Re: [RFC PATCH 18/33] Subarch support for CPUID instruction
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To: |
Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> |
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Subject: |
[Xen-devel] Re: [RFC PATCH 18/33] Subarch support for CPUID instruction |
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From: |
Arjan van de Ven <arjan@xxxxxxxxxxxxx> |
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Date: |
Tue, 18 Jul 2006 12:38:48 +0200 |
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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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Thu, 20 Jul 2006 05:16:28 -0700 |
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On Tue, 2006-07-18 at 11:26 +0100, Keir Fraser wrote:
> 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 some degree "Xen" is just a magic type of cpu that can/should have a
quirk to filter these out... it just feels wrong and fragile to me to do
it via a cpuid filter... once you have the "other" filter as I suggested
I bet the need for the cpuid filter just goes away...
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