linux-kernel - Re: [Patch V1 4/7] x86/microcode/core: Take a snapshot before and after applying microcode

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Message-ID: <874judpqqd.ffs@tglx>
Date:   Fri, 02 Dec 2022 20:09:30 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ashok Raj <ashok.raj@...el.com>, Borislav Petkov <bp@...en8.de>
Cc:     X86-kernel <x86@...nel.org>,
        LKML Mailing List <linux-kernel@...r.kernel.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>, alison.schofield@...el.com,
        reinette.chatre@...el.com
Subject: Re: [Patch V1 4/7] x86/microcode/core: Take a snapshot before and
 after applying microcode

On Tue, Nov 29 2022 at 13:08, Ashok Raj wrote:

> The kernel caches features about each CPU's features at boot in an
> x86_capability[] structure. The microcode update takes one snapshot and
> compares it with the saved copy at boot.
>
> However, the capabilities in the boot copy can be turned off as a result of
> certain command line parameters or configuration restrictions. This can
> cause a mismatch when comparing the values before and after the microcode
> update.
>
> microcode_check() is called after an update to report any previously
> cached CPUID bits might have changed due to the update.
>
> Ignore the capabilities recorded at boot. Take a new snapshot before the
> update and compare with a snapshot after the update to eliminate the false
> warning.

Makes sense.

> +static void copy_cpu_caps(struct cpuinfo_x86 *info)
> +{
> +	/* Reload CPUID max function as it might've changed. */
> +	info->cpuid_level = cpuid_eax(0);
> +
> +	/*
> +	 * Copy all capability leafs to pick up the synthetic ones so that
> +	 * memcmp() below doesn't fail on that. The ones coming from CPUID will
> +	 * get overwritten in get_cpu_cap().
> +	 */
> +	memcpy(info->x86_capability, &boot_cpu_data.x86_capability,
> +	       sizeof(info->x86_capability));
> +
> +	get_cpu_cap(info);
> +}
> +
>  /*
>   * The microcode loader calls this upon late microcode load to recheck features,
>   * only when microcode has been updated. Caller holds microcode_mutex and CPU
>   * hotplug lock.
>   */
> -static void microcode_check(void)
> +static void microcode_check(struct cpuinfo_x86 *orig)
>  {
>  	struct cpuinfo_x86 info;
>  
> @@ -446,15 +462,13 @@ static void microcode_check(void)
>  	info.cpuid_level = cpuid_eax(0);
>  
>  	/*
> -	 * Copy all capability leafs to pick up the synthetic ones so that
> -	 * memcmp() below doesn't fail on that. The ones coming from CPUID will
> -	 * get overwritten in get_cpu_cap().
> -	 */
> -	memcpy(&info.x86_capability, &boot_cpu_data.x86_capability, sizeof(info.x86_capability));
> +	* Copy all capability leafs to pick up the synthetic ones so that
> +	* memcmp() below doesn't fail on that. The ones coming from CPUID will
> +	* get overwritten in get_cpu_cap().
> +	*/
> +	copy_cpu_caps(&info);
>  
> -	get_cpu_cap(&info);
> -
> -	if (!memcmp(&info.x86_capability, &boot_cpu_data.x86_capability,
> +	if (!memcmp(&info.x86_capability, &orig->x86_capability,
>  		    sizeof(info.x86_capability)))
>  		return;
>  
> @@ -469,6 +483,7 @@ static void microcode_check(void)
>  static int microcode_reload_late(void)
>  {
>  	int old = boot_cpu_data.microcode, ret;
> +	struct cpuinfo_x86 info;
>  
>  	pr_err("Attempting late microcode loading - it is dangerous and taints the kernel.\n");
>  	pr_err("You should switch to early loading, if possible.\n");
> @@ -476,9 +491,10 @@ static int microcode_reload_late(void)
>  	atomic_set(&late_cpus_in,  0);
>  	atomic_set(&late_cpus_out, 0);
>  
> +	copy_cpu_caps(&info);
>  	ret = stop_machine_cpuslocked(__reload_late, NULL, cpu_online_mask);

You clearly ran out of newlines and comments here.

>  	if (ret == 0)
> -		microcode_check();
> +		microcode_check(&info);
>  
>  	pr_info("Reload completed, microcode revision: 0x%x -> 0x%x\n",
>  		old, boot_cpu_data.microcode);

Unrelated to that patch, but it just caught my attention. Why are we
printing this is case of failure?

Thanks,

        tglx