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Message-ID: <17fd6bd4a159cc23f20e8e3524f0cdcb871b434d.camel@intel.com>
Subject: Re: [PATCH v3 2/3] x86: Remove vendor checks from
 prefer_mwait_c1_over_halt
From:   Zhang Rui <rui.zhang@intel.com>
To:     Wyes Karny <wyes.karny@amd.com>,
        Dave Hansen <dave.hansen@intel.com>,
        linux-kernel@vger.kernel.org
Cc:     Lewis.Carroll@amd.com, Mario.Limonciello@amd.com,
        gautham.shenoy@amd.com, Ananth.Narayan@amd.com, bharata@amd.com,
        len.brown@intel.com, x86@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        hpa@zytor.com, peterz@infradead.org, chang.seok.bae@intel.com,
        keescook@chromium.org, metze@samba.org, zhengqi.arch@bytedance.com,
        mark.rutland@arm.com, puwen@hygon.cn, rafael.j.wysocki@intel.com,
        andrew.cooper3@citrix.com, jing2.liu@intel.com,
        jmattson@google.com, pawan.kumar.gupta@linux.intel.com
Date:   Wed, 25 May 2022 15:20:48 +0800
In-Reply-To: <f1872469-8a4d-5300-2367-0f56fafcb0de@amd.com>
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         <410f36ccecb36644e196d71fef6e46bdc186b409.camel@intel.com>
         <f1872469-8a4d-5300-2367-0f56fafcb0de@amd.com>
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Hi, Wyes,

On Mon, 2022-05-23 at 21:19 +0530, Wyes Karny wrote:
> Hello Rui,
> 
> On 5/20/2022 7:13 PM, Zhang Rui wrote:
> > On Thu, 2022-05-19 at 09:00 -0700, Dave Hansen wrote:
> > > On 5/10/22 03:18, Wyes Karny wrote:
> > > >  static int prefer_mwait_c1_over_halt(const struct cpuinfo_x86
> > > > *c)
> > > >  {
> > > > +	u32 eax, ebx, ecx, edx;
> > > > +
> > > >  	/* User has disallowed the use of MWAIT. Fallback to
> > > > HALT */
> > > >  	if (boot_option_idle_override == IDLE_NOMWAIT)
> > > >  		return 0;
> > > >  
> > > > -	if (c->x86_vendor != X86_VENDOR_INTEL)
> > > > +	/* MWAIT is not supported on this platform. Fallback to
> > > > HALT */
> > > > +	if (!cpu_has(c, X86_FEATURE_MWAIT))
> > > >  		return 0;
> > 
> > I'm new to x86 code, a dumb question, what about the other vendors?
> > with this patch, prefer_mwait_c1_over_halt() can return 1 for other
> > vendors as well?
> 
> This decision tree is based on cpuid features, so if the processor
> advertises MWAIT C1 support we would be choosing MWAIT.
> If any vendor wants to avoid choosing MWAIT (even cpuid shows MWAIT
> C1
> support), they can explicitly mention. Will add exceptions form them.
> 
That's my concern.

This patch indeed changes the behaviors for vendors other than Intel
and AMD.
I don't know the impact of this, but just point out a behavior change
caused by this patch.

thanks,
rui