linux-kernel - Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

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Message-ID: <5529aa92-191d-4120-a005-28fe5c209a4f@zytor.com>
Date: Mon, 23 Jun 2025 21:57:13 -0700
From: Xin Li <xin@...or.com>
To: "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Poimboeuf
 <jpoimboe@...nel.org>,
        Xiongwei Song <xiongwei.song@...driver.com>,
        Xin Li <xin3.li@...el.com>, "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Brijesh Singh <brijesh.singh@....com>,
        Michael Roth <michael.roth@....com>, Tony Luck <tony.luck@...el.com>,
        Alexey Kardashevskiy <aik@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>, Sohil Mehta <sohil.mehta@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        Kai Huang <kai.huang@...el.com>, Sandipan Das <sandipan.das@....com>,
        Breno Leitao <leitao@...ian.org>,
        Rick Edgecombe
 <rick.p.edgecombe@...el.com>,
        Alexei Starovoitov <ast@...nel.org>, Hou Tao <houtao1@...wei.com>,
        Juergen Gross <jgross@...e.com>,
        Vegard Nossum <vegard.nossum@...cle.com>, Kees Cook <kees@...nel.org>,
        Eric Biggers <ebiggers@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>,
        "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Luis Chamberlain <mcgrof@...nel.org>, Yuntao Wang <ytcoode@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Tejun Heo <tj@...nel.org>, Changbin Du <changbin.du@...wei.com>,
        Huang Shijie <shijie@...amperecomputing.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Namhyung Kim <namhyung@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        linux-mm@...ck.org, Yian Chen <yian.chen@...el.com>
Subject: Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

On 6/23/2025 7:04 PM, H. Peter Anvin wrote:
> On June 23, 2025 10:40:59 AM PDT, Xin Li <xin@...or.com> wrote:
>> On 6/20/2025 5:50 PM, H. Peter Anvin wrote:
>>> On 2025-06-20 17:45, H. Peter Anvin wrote:
>>>>>
>>>>> But I simply hate adding a disabled feature that depends on !X86_64;
>>>>> x86_64 has a broad scope, and new CPU features are often intentionally
>>>>> not enabled for 32-bit.
>>>>>
>>>>> (X86_DISABLED_FEATURE_PCID is the only one before LASS)
>>>>
>>>> More importantly, it is wrong.
>>>>
>>>> The 32-bit build can depend on this feature not existing, therefore it SHOULD be listed as a disabled feature.
>>>>
>>>
>>> Ok, that was word salad. What I meant was that the original patch is correct, and we SHOULD have this as a disabled feature.
>>
>> Agreed!
>>
>>> The reason is that it reduces the need to explicitly test for 32/64 bits for features that don't exist on 32 bits. When they are flagged as disabled, they get filtered out *at compile time*.
>>
>> It's better to make it depend on X86_32 directly rather than !X86_64:
>>
>> config X86_DISABLED_FEATURE_LASS
>> 	def_bool y
>> 	depends on X86_32
>>
>>
>> But the disabled feature list due to lack of 32-bit enabling will keep
>> growing until we remove 32-bit kernel code.
>>
>> Wondering should we bother enforcing cpuid_deps[] on 32-bit?
>>
>> IOW, turn off the feature when its dependency isn’t satisfied on 32b-it;
>> don’t just throw a warning and hope for the best.
>>
>> Thanks!
>>     Xin
>>
> 
> We should have the dependencies enforced; in fact, preferably we would enforce them at build time as well.
> 
> 

Yeah, sounds something we can do later :)