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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>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Carroll, Lewis" <Lewis.Carroll@amd.com>,
        "Shenoy, Gautham Ranjal" <gautham.shenoy@amd.com>,
        "Narayan, Ananth" <Ananth.Narayan@amd.com>,
        "Rao, Bharata Bhasker" <bharata@amd.com>,
        "len.brown@intel.com" <len.brown@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "chang.seok.bae@intel.com" <chang.seok.bae@intel.com>,
        "keescook@chromium.org" <keescook@chromium.org>,
        "metze@samba.org" <metze@samba.org>,
        "zhengqi.arch@bytedance.com" <zhengqi.arch@bytedance.com>,
        "mark.rutland@arm.com" <mark.rutland@arm.com>
Subject: RE: [PATCH] x86: Prefer MWAIT over HALT on AMD processors
Thread-Topic: [PATCH] x86: Prefer MWAIT over HALT on AMD processors
Thread-Index: AQHYSO0si5fvSUiei0GnFDYbI7mXuKzhWxKAgAARiYCAAAWWwIAABNIAgABQN2CAAFaUAIAA0EgAgAIJqoCAAEOtoIAKt08Q
Date:   Thu, 14 Apr 2022 21:06:40 +0000
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[Public]


> -----Original Message-----
> From: Limonciello, Mario
> Sent: Thursday, April 7, 2022 20:24
> To: Dave Hansen <dave.hansen@intel.com>; Thomas Gleixner
> <tglx@linutronix.de>; Peter Zijlstra <peterz@infradead.org>; Karny, Wyes
> <Wyes.Karny@amd.com>
> Cc: linux-kernel@vger.kernel.org; Carroll, Lewis <Lewis.Carroll@amd.com>;
> Shenoy, Gautham Ranjal <gautham.shenoy@amd.com>; Narayan, Ananth
> <Ananth.Narayan@amd.com>; Rao, Bharata Bhasker <bharata@amd.com>;
> len.brown@intel.com; x86@kernel.org; mingo@redhat.com; bp@alien8.de;
> dave.hansen@linux.intel.com; hpa@zytor.com; chang.seok.bae@intel.com;
> keescook@chromium.org; metze@samba.org;
> zhengqi.arch@bytedance.com; mark.rutland@arm.com
> Subject: RE: [PATCH] x86: Prefer MWAIT over HALT on AMD processors
>=20
> [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.
> >
> > But, the code as written depends on behavior for *all* of family 0x19:
> >
> > >         case 0x17: fallthrough;
> > >         case 0x19: init_amd_zn(c); break;
> >
> > So, while the docs for "family 19h model 01h" are *consistent* with thi=
s
> > code, there's also nothing preventing the docs for "family 19h model
> > 02h" from breaking this new MWAIT code.
> >
> > 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
> honest.
>=20
> Point taken.  As you're probably aware releasing documentation is never a
> quick
> process, but Lewis and Wyes can take this to the powers that be internall=
y to
> see
> what can be done for the future.

This thread did splinter a bit as it was between speculation control and MW=
AIT.
So I just wanted to complete the thoughts related to the speculation as tha=
t came
up.  Having gone back over what is in the outward facing documentation I di=
d want
to share to you that APM volume 2 page 67-70 does include the details
for speculation control and proper architectural discovery of the MSR.
https://www.amd.com/system/files/TechDocs/24593.pdf

>=20
> >
> > What's missing is something that says:
> >
> > 	All AMD family 0x17 and 0x19 that enumerate support for MWAIT
> > 	also support this "early MWAIT" implementation
> >
> > 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.
> >
> > 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.
>=20
> OK... I think the commit message and/or a comment is doable in the short
> term for v2.