linux-kernel - Re: [PATCH v3 05/10] x86/mtrr: split generic_set

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Date:   Mon, 19 Sep 2022 21:25:30 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v3 05/10] x86/mtrr: split generic_set_all()

On Thu, Sep 08, 2022 at 10:49:09AM +0200, Juergen Gross wrote:
> Split generic_set_all() into multiple parts, while moving the main
> function body into cacheinfo.c.
> 
> This prepares the support of PAT without needing MTRR support by

"Prepare PAT support without ... "

> moving the main function body of generic_set_all() into cacheinfo.c
> while renaming it to cache_cpu_init(). The MTRR specific parts are
> moved into a dedicated small function called by cache_cpu_init().
> The PAT and MTRR specific functions are called conditionally based
> on the cache_generic bit settings.
> 
> The setting of smp_changes_mask is merged into the (new) function

... and so on. So the commit message should not say what you're doing -
that should be visible from the diff itself. It should talk more about
the *why* you're doing it.

...

> diff --git a/arch/x86/kernel/cpu/cacheinfo.c b/arch/x86/kernel/cpu/cacheinfo.c
> index 47e2c72fa8a4..36378604ec61 100644
> --- a/arch/x86/kernel/cpu/cacheinfo.c
> +++ b/arch/x86/kernel/cpu/cacheinfo.c
> @@ -1120,3 +1120,22 @@ void cache_enable(void) __releases(cache_disable_lock)
>  
>  	raw_spin_unlock(&cache_disable_lock);
>  }
> +
> +void cache_cpu_init(void)
> +{
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +	cache_disable();
> +
> +	/* Set MTRR state. */

Yeah, and when you name the functions properly as you've done, you don't
really need those comments anymore either.

> +	if (cache_generic & CACHE_GENERIC_MTRR)
> +		mtrr_generic_set_state();
> +
> +	/* Set PAT. */

And this one.

> +	if (cache_generic & CACHE_GENERIC_PAT)
> +		pat_init();
> +
> +	cache_enable();
> +	local_irq_restore(flags);
> +}
> diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
> index 5ed397f03a87..fc7b2d952737 100644
> --- a/arch/x86/kernel/cpu/mtrr/generic.c
> +++ b/arch/x86/kernel/cpu/mtrr/generic.c
> @@ -731,30 +731,19 @@ void mtrr_enable(void)
>  	mtrr_wrmsr(MSR_MTRRdefType, deftype_lo, deftype_hi);
>  }
>  
> -static void generic_set_all(void)
> +void mtrr_generic_set_state(void)
>  {
>  	unsigned long mask, count;
> -	unsigned long flags;
> -
> -	local_irq_save(flags);
> -	cache_disable();
>  
>  	/* Actually set the state */
>  	mask = set_mtrr_state();
>  
> -	/* also set PAT */
> -	pat_init();
> -
> -	cache_enable();
> -	local_irq_restore(flags);
> -
>  	/* Use the atomic bitops to update the global mask */
>  	for (count = 0; count < sizeof(mask) * 8; ++count) {
>  		if (mask & 0x01)
>  			set_bit(count, &smp_changes_mask);
>  		mask >>= 1;
>  	}
> -
>  }
>  
>  /**
> @@ -854,7 +843,7 @@ int positive_have_wrcomb(void)
>   * Generic structure...
>   */
>  const struct mtrr_ops generic_mtrr_ops = {
> -	.set_all		= generic_set_all,
> +	.set_all		= cache_cpu_init,

I was gonna say that this looks weird - a set_all function pointer
assigned to a init function but that changes in the next patch.

But yeah, I like where this is going.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette