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

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Date:   Tue, 5 Apr 2022 08:10:05 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Wyes Karny <wyes.karny@....com>
Cc:     linux-kernel@...r.kernel.org, Lewis.Carroll@....com,
        Mario.Limonciello@....com, gautham.shenoy@....com,
        Ananth.Narayan@....com, bharata@....com, len.brown@...el.com,
        x86@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        chang.seok.bae@...el.com, keescook@...omium.org, metze@...ba.org,
        zhengqi.arch@...edance.com, mark.rutland@....com
Subject: Re: [PATCH] x86: Prefer MWAIT over HALT on AMD processors

On 4/5/22 07:07, Peter Zijlstra wrote:
> On Tue, Apr 05, 2022 at 06:30:21PM +0530, Wyes Karny wrote:
>> +static inline bool early_mwait_supported(const struct cpuinfo_x86 *c)
>> +{
>> +	if (c->x86_vendor == X86_VENDOR_INTEL)
>> +		return true;
>> +
>> +	if (c->x86_vendor == X86_VENDOR_AMD && cpu_has(c, X86_FEATURE_ZEN))
> What about Hygon? For some reason you guys don't co-ordinate and we end
> up getting endless 'make-same' patches, sometimes separated by years :/

Believe it or not Hygon seems to work OK with this because:

> static void init_hygon(struct cpuinfo_x86 *c)
> {
...
>         set_cpu_cap(c, X86_FEATURE_ZEN);

I do worry a bit though that using X86_FEATURE_ZEN is going to bite us
long-term.  It currently claims to be set for "family 0x17 or above":

> #define X86_FEATURE_ZEN                 ( 7*32+28) /* "" CPU is AMD family 0x17 or above (Zen) */

But then it goes and gets used in side-channel defense:

>         if (!static_cpu_has(X86_FEATURE_ZEN)) {
>                 msr |= ssbd_tif_to_amd_ls_cfg(tifn);
>                 wrmsrl(MSR_AMD64_LS_CFG, msr);
>                 return;
>         }

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?