linux-kernel - Re: [PATCH v2 1/4] x86/split_lock: Enumerate #AC exception for split locked access feature

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Date:   Fri, 29 Jun 2018 18:23:35 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...el.com>
cc:     Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...hat.com>,
        H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 1/4] x86/split_lock: Enumerate #AC exception for split
 locked access feature

On Fri, 29 Jun 2018, Dave Hansen wrote:
> On 06/29/2018 07:33 AM, Fenghua Yu wrote:
> > +/* Detect feature of #AC for split lock by probing bit 29 in MSR_TEST_CTL. */
> > +void detect_ac_split_lock(void)
> > +{
> > +	u64 val, orig_val;
> > +	int ret;
> > +
> > +	/* Attempt to read the MSR. If the MSR doesn't exist, reading fails. */
> > +	ret = rdmsrl_safe(MSR_TEST_CTL, &val);
> > +	if (ret)
> > +		return;
> 
> This is a bit fast and loose with how the feature is detected, which
> might be OK, but can we call out why we are doing this, please?
> 
> Is this MSR not really model-specific?  Is it OK to go poking at it on
> all x86 variants?  Or, do we at _least_ need a check for Intel cpus in here?

That definitely needs a vendor check. Also the whole code needs to be
compiled out if CONFIG_INTEL=n.

Aside of that this wants to be enumerated. CPUID or MISC_FEATURES and not
this guess work detection logic. Why do I have to ask for that for every
other new feature thingy?

And pretty please, can we either stick that stuff into cpu/intel.c or if it
really needs a separate file avoid that horrible test_ctl.c file name? 

Thanks,

	tglx