linux-kernel - Re: [PATCH v3 4/4] x86/cpufeatures: Use AWK to generate {REQUIRED|DISABLED}_MASK_BIT

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Date: Mon, 24 Jun 2024 11:24:33 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Xin Li <xin@...or.com>, Brian Gerst <brgerst@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, will@...nel.org,
 peterz@...radead.org, akpm@...ux-foundation.org, acme@...nel.org,
 namhyung@...nel.org
Subject: Re: [PATCH v3 4/4] x86/cpufeatures: Use AWK to generate
 {REQUIRED|DISABLED}_MASK_BIT_SET

On 24/06/2024 8:29 am, Xin Li wrote:
> On 6/23/2024 1:28 PM, Brian Gerst wrote:
>> On Sat, Jun 22, 2024 at 1:14 PM Xin Li (Intel) <xin@...or.com> wrote:
>>> -               printf "#define %s_MASK_CHECK
>>> BUILD_BUG_ON_ZERO(NCAPINTS != %d)\n\n", s, ncapints;
>>> +               printf "\n#define %s_FEATURE(x)\t\t\t\t\\\n", s;
>>> +               printf "\t((\t\t\t\t\t";
>>> +               for (i = 0; i < ncapints; i++) {
>>> +                       if (masks[i])
>>> +                               printf "\t\\\n\t\t((x) >> 5) == %2d
>>> ? %s_MASK%d :", i, s, i;
>>> +               }
>>> +               printf " 0\t\\\n";
>>> +               printf "\t) & (1 << ((x) & 31)))\n";
>>
>> The original macro had 1UL here.  I don't know if it is strictly
>> necessary in this case since we're using 32-bit masks, but it would
>> probably be safer.
>
> I did notice it, but don't think UL is needed.

You do need 1U, or you'll trip UBSAN every time you test feature 31 in a
word.

I'll note that the hypervisor bit is one such example.

~Andrew