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Re: [Qemu-ppc] [RFC PATCH] powerpc: add PVR mask support

From: Benjamin Herrenschmidt
Subject: Re: [Qemu-ppc] [RFC PATCH] powerpc: add PVR mask support
Date: Thu, 15 Aug 2013 16:28:18 +1000

On Thu, 2013-08-15 at 08:10 +0200, Alexander Graf wrote:

> So you're saying it's good to remove a well established feature on 5% 
> of the supported CPUs, leave the others inconsistent with the change 
> and then declare the whole thing an improvement?

WTF are you talking about ?

To need an exact PVR definition to the last bit means every time a minor
chip rev comes out of IBM, KVM will stop working until qemu is updated
to know about that revision.

This is dumb.

Being able to emulate a P7 2.1 vs a P7 2.3 is completely pointless since
essentially they expose the same architecture and the bugs that are
fixed between those revisions are for the most part not guest visible
nor even emulated by qemu to begin with.

Now there *might* be some value in being able to specify among "known
supported" versions for things like P5 (but frankly, who gives a damn ?
Who is actually going to *use* that ? Nobody really ....)

In that case it's easy ... have a name match with the table entry.

With the mask & value, you can do like the kernel, ie, have first in the
list the specific entries you want to match against (ie, P7_2_1,
P7_2_3, ...) and fallback to a generic "P7 any revision" entry. That way
qemu will still work if IBM releases a P7 v2.4 you don't know about.

As for selecting it, similarly, you can do an exact match on the name
(or a partial match as a fallback, I don't care) and pickup the PVR
value out of the table for emulation.

Point is, what we have now is crap. This is the best fix I've seen so
far. It's useful, cover the 99.9% of the possible use cases I can think
of, but you seem to care more about hypothetical scenario that have no
practical interest on the field.


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