linux-kernel - RE: [PATCH] x86: Prefer MWAIT over HALT on AMD processors

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Message-ID: <BL1PR12MB5157E978932A02865541D319E2E99@BL1PR12MB5157.namprd12.prod.outlook.com>
Date:   Fri, 8 Apr 2022 01:24:15 +0000
From:   "Limonciello, Mario" <Mario.Limonciello@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Karny, Wyes" <Wyes.Karny@....com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Carroll, Lewis" <Lewis.Carroll@....com>,
        "Shenoy, Gautham Ranjal" <gautham.shenoy@....com>,
        "Narayan, Ananth" <Ananth.Narayan@....com>,
        "Rao, Bharata Bhasker" <bharata@....com>,
        "len.brown@...el.com" <len.brown@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "chang.seok.bae@...el.com" <chang.seok.bae@...el.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "metze@...ba.org" <metze@...ba.org>,
        "zhengqi.arch@...edance.com" <zhengqi.arch@...edance.com>,
        "mark.rutland@....com" <mark.rutland@....com>
Subject: RE: [PATCH] x86: Prefer MWAIT over HALT on AMD processors

[Public]

> 
> 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 this
> 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.

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 internally to see
what can be done for the future.

> 
> 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.

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