linux-kernel - Re: [PATCH 13/15] x86/cpu: Make CONFIG_X86

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Message-Id: <1d4ddcab-cf46-4d7e-9e33-de12b6bd350c@app.fastmail.com>
Date: Fri, 25 Apr 2025 23:54:15 +0200
From: "Arnd Bergmann" <arnd@...nel.org>
To: "H. Peter Anvin" <hpa@...or.com>, "Ingo Molnar" <mingo@...nel.org>,
 linux-kernel@...r.kernel.org
Cc: "Ahmed S . Darwish" <darwi@...utronix.de>,
 "Andrew Cooper" <andrew.cooper3@...rix.com>,
 "Ard Biesheuvel" <ardb@...nel.org>, "Borislav Petkov" <bp@...en8.de>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>,
 "John Ogness" <john.ogness@...utronix.de>,
 "Linus Torvalds" <torvalds@...ux-foundation.org>,
 "Peter Zijlstra" <peterz@...radead.org>,
 "Thomas Gleixner" <tglx@...utronix.de>
Subject: Re: [PATCH 13/15] x86/cpu: Make CONFIG_X86_CX8 unconditional

On Fri, Apr 25, 2025, at 17:15, H. Peter Anvin wrote:
> On April 25, 2025 5:10:27 AM PDT, Arnd Bergmann <arnd@...nel.org> wrote:
>>
>>I just noticed this one: the final 'default "4"' is no longer possible
>>here and can be removed. All the remaining CPUs report family "5" or
>>higher.
>>
>>There is an old issue for some rare CPUs (Geode LX and Crusoe) that
>>support CMOV but report family=6. These to boot a kernel with X86_MINIMUM_CPU_FAMILY=6 because it triggers the boot time check.
>>
>
> They report family=5 because family=6 implies fcomi and nopl support 
> (in the case of Crusoe, they have fcomi but didn't support movl.)

Ah right, I see now. I had only checked that the kernel itself
no longer uses nopl after your ba0593bf553c ("x86: completely
disable NOPL on 32 bits"), and I had seen that Debian intentionally
builds 32-bit i686 kernels with CONFIG_MGEODEGX1.

I now found that both Debian 12 and gcc 11 changed their definition
if 686 to actually require nopl for Indirect branch tracking 
(-fcf-protection) in user space, as discussed in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104713

So even if it makes sense for GeodeLX specific kernel to use CMOV,
any general-purpose i686 distro would still want to enable IBT
in userspace to gain IBT on Tiger Lake and newer 64-bit CPUs.

     Arnd