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Peter Anvin" , "Ingo Molnar" , linux-kernel@vger.kernel.org Cc: "Ahmed S . Darwish" , "Andrew Cooper" , "Ard Biesheuvel" , "Borislav Petkov" , "Dave Hansen" , "John Ogness" , "Linus Torvalds" , "Peter Zijlstra" , "Thomas Gleixner" Message-Id: <1d4ddcab-cf46-4d7e-9e33-de12b6bd350c@app.fastmail.com> In-Reply-To: <8D770F85-5417-4A9E-80DE-1B6A890DECEF@zytor.com> References: <20250425084216.3913608-1-mingo@kernel.org> <20250425084216.3913608-14-mingo@kernel.org> <956412a3-43c2-4d6e-bea2-2573c98233ae@app.fastmail.com> <8D770F85-5417-4A9E-80DE-1B6A890DECEF@zytor.com> Subject: Re: [PATCH 13/15] x86/cpu: Make CONFIG_X86_CX8 unconditional Content-Type: text/plain Content-Transfer-Encoding: 7bit On Fri, Apr 25, 2025, at 17:15, H. Peter Anvin wrote: > On April 25, 2025 5:10:27 AM PDT, Arnd Bergmann 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