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From:   "Limonciello, Mario" <Mario.Limonciello@amd.com>
To:     Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        "Karny, Wyes" <Wyes.Karny@amd.com>
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        "Carroll, Lewis" <Lewis.Carroll@amd.com>,
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Subject: RE: [PATCH] x86: Prefer MWAIT over HALT on AMD processors
Thread-Topic: [PATCH] x86: Prefer MWAIT over HALT on AMD processors
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Date:   Fri, 8 Apr 2022 01:24:15 +0000
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[Public]

>=20
> I don't want to give you too hard of a time on this.  But, that's not
> architecture, that's just telling folks what the implementation is on
> *one* CPU model.  Don't get me wrong: these model-specific docs are
> great, and I wish Intel published something like that.
>=20
> But, the code as written depends on behavior for *all* of family 0x19:
>=20
> >         case 0x17: fallthrough;
> >         case 0x19: init_amd_zn(c); break;
>=20
> So, while the docs for "family 19h model 01h" are *consistent* with this
> code, there's also nothing preventing the docs for "family 19h model
> 02h" from breaking this new MWAIT code.
>=20
> Now, AMD is full of smart folks that aren't going out of their way to
> try and break existing software.  But, actual documentation of the
> *architecture* is really preferable to what we have now.  It helps
> establish a shared hardware/software contract that keeps both sides hones=
t.

Point taken.  As you're probably aware releasing documentation is never a q=
uick
process, but Lewis and Wyes can take this to the powers that be internally =
to see
what can be done for the future.

>=20
> What's missing is something that says:
>=20
> 	All AMD family 0x17 and 0x19 that enumerate support for MWAIT
> 	also support this "early MWAIT" implementation
>=20
> That represents a promise from AMD that this can't break in the future.
>  It both gives us something unambiguous to write code with *and*
> something to help nudge our hardware and microcode colleagues if they
> start to do something funny in the future.
>=20
> Just having that in the changelog would be fine, and a genuine
> improvement over what we have now.  Having it one of the
> model-independent architecture manuals would be even better.

OK... I think the commit message and/or a comment is doable in the short
term for v2.