Message-ID: <Z8resWzgtZaTuzEN@gmail.com>
Date: Fri, 7 Mar 2025 12:55:29 +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 v5] x86/cpufeature: Warn about unmet feature dependencies


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

> Currently, the cpuid_deps[] table is only exercised when a particular
> feature gets explicitly disabled and clear_cpu_cap() is called. However,
> some of these listed dependencies might already be missing during boot.
> 
> These types of errors shouldn't generally happen in production
> environments but they could sometimes sneak through, especially when VMs
> and Kconfigs are in the mix. Also, the kernel might introduce artificial
> dependencies between unrelated features such as making LAM depend on
> LASS.
> 
> Unexpected failures can occur when the kernel tries to use such a
> feature. Add a simple boot time scan of the cpuid_deps[] table to detect
> the missing dependencies. One option is to disable all of such features
> during boot but that may cause regressions in existing systems. For now,
> just warn about the missing dependencies to create awareness.
> 
> As a trade-off between spamming the kernel log and keeping track of all
> the features that have been warned about, only warn about the first
> missing dependency. Any subsequent unmet dependency will only be logged
> after the first one has been resolved.
> 
> Features are typically represented through unsigned integers within the
> kernel though some of them have user friendly names if they are exposed
> via cpuinfo.  Show the friendlier name if available otherwise display
> the X86_FEATURE_* numerals to make it easier to identify the feature.
> 
> Suggested-by: Tony Luck <tony.luck@...el.com>
> Suggested-by: Ingo Molnar <mingo@...hat.com>
> Signed-off-by: Sohil Mehta <sohil.mehta@...el.com>
> ---
> v5: Don't disable features with unmet dependencies (Ingo)
>     Use pr_warn_once() for the warning (Ingo)
> 
> v4: https://lore.kernel.org/lkml/20241210224037.3052555-1-sohil.mehta@intel.com/
>     Update the log level to pr_info() (Ingo)
>     Update the char buffer size to 16 (0day warning)
>     Dropped Dave's review tag since the code has changed a bit more
>     than usual.
> 
> v3: Picked up the review tag.
>     Reworded the commit message.
> 
> v2: Use cpu_has() instead of boot_cpu_has() (Sean)
> ---
>  arch/x86/include/asm/cpufeature.h |  1 +
>  arch/x86/kernel/cpu/common.c      |  4 ++++
>  arch/x86/kernel/cpu/cpuid-deps.c  | 35 +++++++++++++++++++++++++++++++
>  3 files changed, 40 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
> index de1ad09fe8d7..1bb30e82ea77 100644
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -149,6 +149,7 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
>  
>  extern void setup_clear_cpu_cap(unsigned int bit);
>  extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
> +void scan_feature_dependencies(struct cpuinfo_x86 *c);
>  
>  #define setup_force_cpu_cap(bit) do {			\
>  							\
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 7cce91b19fb2..52111d97c8ae 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1610,6 +1610,7 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>  
>  		c->cpu_index = 0;
>  		filter_cpuid_features(c, false);
> +		scan_feature_dependencies(c);
>  
>  		if (this_cpu->c_bsp_init)
>  			this_cpu->c_bsp_init(c);
> @@ -1870,6 +1871,9 @@ static void identify_cpu(struct cpuinfo_x86 *c)
>  	/* Filter out anything that depends on CPUID levels we don't have */
>  	filter_cpuid_features(c, true);
>  
> +	/* Scan for unmet dependencies based on the CPUID dependency table */
> +	scan_feature_dependencies(c);

s/scane_feature_dependencies
 /x86_check_cpufeature_deps

or so.

> +/*
> + * Return the feature "name" if available otherwise return
> + * the X86_FEATURE_* numerals to make it easier to identify
> + * the feature.

s/available otherwise
 /available, otherwise

> + */
> +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 scan_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)) {
> +			/*
> +			 * Only warn about the first unmet dependency on the
> +			 * first CPU where it is encountered to avoid spamming
> +			 * the kernel log.
> +			 */
> +			pr_warn_once("CPU%d: feature:%s may not work properly without feature:%s\n",
> +				     smp_processor_id(),
> +				     x86_feature_name(d->feature, feature_buf),
> +				     x86_feature_name(d->depends, depends_buf));

I'd make this a bit less passive-aggressive, something like:

     x86 CPU feature dependency check failure: CPU%d has '%s' enabled but '%s' disabled. Kernel might be fine, but no guarantees.

Because initially most of these warnings will be about quirks and 
oddities that happen to work fine in the current code.

Thanks,

	Ingo

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