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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <f3dbf693-8c88-4736-9ec9-83bedc097c2b@zytor.com>
Date: Mon, 23 Jun 2025 22:11:08 -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
Subject: Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

On 6/23/2025 9:57 PM, Xin Li wrote:
> 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 :)
> 

We could introduce a new Kconfig file at

     arch/x86/Kconfig.cpufeatures.disabled_on_32bit

to track all features disabled due to lack of 32-bit support.