linux-kernel - Re: [PATCH v8 09/33] x86/cpu: add X86_CR4

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Date:   Mon, 05 Jun 2023 14:01:41 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Xin Li <xin3.li@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, kvm@...r.kernel.org
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, peterz@...radead.org, andrew.cooper3@...rix.com,
        seanjc@...gle.com, pbonzini@...hat.com, ravi.v.shankar@...el.com,
        jiangshanlai@...il.com, shan.kang@...el.com
Subject: Re: [PATCH v8 09/33] x86/cpu: add X86_CR4_FRED macro

On Mon, Apr 10 2023 at 01:14, Xin Li wrote:

> From: "H. Peter Anvin (Intel)" <hpa@...or.com>
>
> Add X86_CR4_FRED macro for the FRED bit in %cr4. This bit should be a

s/should/must/ no?

> +/*
> + * These bits should not change their value after CPU init is finished.
> + * The explicit cast to unsigned long suppresses a warning on i386 for
> + * x86-64 only feature bits >= 32.
> + */
>  static const unsigned long cr4_pinned_mask =
> -	X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
> -	X86_CR4_FSGSBASE | X86_CR4_CET;
> +	(unsigned long)

That type cast is required because:

+#define X86_CR4_FRED		_BITULL(X86_CR4_FRED_BIT)

Bah. Fred is 64 bit only. So why defining this as 1ULL << 32
unconditionally and stripping the bit off on 32bit via the type cast?

#ifdef CONFIG_X86_64
#define X86_CR4_FRED		_BITUL(X86_CR4_FRED_BIT)
#else
#define X86_CR4_FRED		0
#endif

would be too obvious, right?

> +	(X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
> +	 X86_CR4_FSGSBASE | X86_CR4_CET | X86_CR4_FRED);

Thanks,

        tglx