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

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Date:   Mon, 5 Jun 2023 10:19:10 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Thomas Gleixner <tglx@...utronix.de>, 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,
        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 6/5/23 05:01, Thomas Gleixner wrote:
> 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?
> 

It also adds an #ifdef mess to avoid a simple typecast. Is that the 
right tradeoff?

I'm not saying it is or it isn't, it is an open question.

	-hpa