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

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Message-ID: <87a6cz0yvh.ffs@tglx>
Date:   Wed, 06 Apr 2022 03:44:18 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Limonciello, Mario" <Mario.Limonciello@....com>,
        Dave Hansen <dave.hansen@...el.com>,
        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

Mario,

On Tue, Apr 05 2022 at 20:40, Mario Limonciello wrote:

> [Public]

Really useful information that your post is public while some of the
earlier posts in this _public_ thread were marked '[AMD confidential]'.

>> >> This seem _bit_ at odds with the commit message (and the AMD SSBD
>> >> whitepaper):
>> >>
>> >>>     Add the necessary synchronization logic for AMD family 17H.
>> >> So, is X86_FEATURE_ZEN for family==0x17, or family>=0x17?
>> > There are Zen family CPUs and APUs from family 0x19.  Perhaps at the
>> > time of the whitepaper there weren't yet though.
>> 
>> Is this a gap in the documentation, then?  Is there some documentation
>> of the availability of SSBD mitigations on family 0x19?
>
> It looks like a misinterpretation of the document.

Not at all. The document does not mention family 19h at all. So where is
the misinterpretation?

Dave was asking for documentation for family 0x19, right?

> Notice it mentions in Non-architectural MSRs explicitly:
>
> "some models of family 17h have logic that allow loads to bypass older stores 
> but lack the architectural SPEC_CTRL or VIRT_SPEC_CTR"

That's relevant to Dave's question in which way? 

> But that is not for all family 17h nor for family 19h.  You can see earlier in
> the document the method to detect presence for SSBD which applies to the
> rest of 17h and 19h.

We know how to read this document. But this document does not mention
anything else than family 17h. So what are you talking about?

> That code in amd_set_core_ssb_state is only used for one of the
> mitigation codepaths without MSR support, not for all Zen CPUs.

Again, how is that relevant to the legitimate question whether that
document applies to family 17h only or to 17h+ which includes 19h?

We need to make a decison about what X86_FEATURE_ZEN means. Is it that
hard to give an authoritive answer?

Thanks,

        tglx