linux-kernel - Re: [PATCH v4 06/12] x86/mtrr: replace vendor tests in MTRR code

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Message-ID: <20230324165611.GIZB3WK13NdjceLWnN@fat_crate.local>
Date:   Fri, 24 Mar 2023 17:56:11 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.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 v4 06/12] x86/mtrr: replace vendor tests in MTRR code

On Mon, Mar 06, 2023 at 05:34:19PM +0100, Juergen Gross wrote:
> Modern CPUs all share the same MTRR interface implemented via
> generic_mtrr_ops.
> 
> At several places in MTRR code this generic interface is deduced via
> is_cpu(INTEL) tests, which is only working due to X86_VENDOR_INTEL
> being 0 (the is_cpu() macro is testing mtrr_if->vendor, which isn't
> explicitly set in generic_mtrr_ops).
> 
> Fix that by replacing the is_cpu(INTEL) tests with testing for mtrr_if
> to be &generic_mtrr_ops.

Two things:

* is_cpu() checks also whether mtrr_if is set. And we don't set it for
all vendors. I wanted to replace that thing with a vendor check recently
but there's that little issue.

I guess for the cases where we have the generic MTRR implementation, we
can safely assume that mtrr_if is set. Which leads me to the second
thing:

* If you're going to test for &generic_mtrr_ops, then you can just as
well do

	cpu_feature_enabled(X86_FEATURE_MTRR)

which is a lot more telling.

> diff --git a/arch/x86/kernel/cpu/mtrr/mtrr.c b/arch/x86/kernel/cpu/mtrr/mtrr.c
> index 5fe62ee0361b..0c83990501f5 100644
> --- a/arch/x86/kernel/cpu/mtrr/mtrr.c
> +++ b/arch/x86/kernel/cpu/mtrr/mtrr.c
> @@ -108,14 +108,12 @@ static int have_wrcomb(void)
>  /*  This function returns the number of variable MTRRs  */
>  static void __init set_num_var_ranges(bool use_generic)
>  {
> -	unsigned long config = 0, dummy;
> +	unsigned long config, dummy;
>  
>  	if (use_generic)
>  		rdmsr(MSR_MTRRcap, config, dummy);
> -	else if (is_cpu(AMD) || is_cpu(HYGON))
> -		config = 2;
> -	else if (is_cpu(CYRIX) || is_cpu(CENTAUR))
> -		config = 8;
> +	else
> +		config = mtrr_if->var_regs;
>  
>  	num_var_ranges = config & 0xff;
>  }

Since you're touching this function, you might simply expand its body in
its only call site in mtrr_bp_init(), put a comment above the expanded
code and remove that function.

That is, if we're going to do the ->var_regs thing.

Thx.

-- 
Regards/Gruss,
    Boris.

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