linux-kernel - Re: [PATCH v4] x86/cpufeature: Add feature dependency checks

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Message-ID: <Z8Cy-IonwmCGNkkQ@gmail.com>
Date: Thu, 27 Feb 2025 19:46:16 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Sohil Mehta <sohil.mehta@...el.com>
Cc: x86@...nel.org, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H . Peter Anvin" <hpa@...or.com>, Uros Bizjak <ubizjak@...il.com>,
	Sandipan Das <sandipan.das@....com>,
	Sean Christopherson <seanjc@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Tony Luck <tony.luck@...el.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Nikolay Borisov <nik.borisov@...e.com>,
	Eric Biggers <ebiggers@...gle.com>, Xin Li <xin3.li@...el.com>,
	Alexander Shishkin <alexander.shishkin@...el.com>,
	Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] x86/cpufeature: Add feature dependency checks


* Sohil Mehta <sohil.mehta@...el.com> wrote:

> +
> +/*
> + * Return the feature "name" if available otherwise return
> + * the X86_FEATURE_* numerals to make it easier to identify
> + * the feature.
> + */
> +static const char *x86_feature_name(unsigned int feature, char *buf)
> +{
> +	if (x86_cap_flags[feature])
> +		return x86_cap_flags[feature];
> +
> +	snprintf(buf, 16, "%d*32+%2d", feature / 32, feature % 32);
> +
> +	return buf;
> +}
> +
> +void filter_feature_dependencies(struct cpuinfo_x86 *c)
> +{
> +	char feature_buf[16], depends_buf[16];
> +	const struct cpuid_dep *d;
> +
> +	for (d = cpuid_deps; d->feature; d++) {
> +		if (cpu_has(c, d->feature) && !cpu_has(c, d->depends)) {
> +			pr_info("CPU%d: Disabling feature %s due to missing feature %s\n",
> +				smp_processor_id(),
> +				x86_feature_name(d->feature, feature_buf),
> +				x86_feature_name(d->depends, depends_buf));
> +			do_clear_cpu_cap(c, d->feature);
> +		}
> +	}

So let's not disable any CPU features actively for the time being, how 
about issuing a pr_warn() only about the dependency violation?

I think the main problem is when these problems slip through 100% 
unnoticed.

Thanks,

	Ingo