linux-kernel - Re: [PATCH v2] x86: Clear feature bits disabled at compile-time

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Message-ID: <2025072347-legible-running-efbb@gregkh>
Date: Wed, 23 Jul 2025 13:57:34 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
Cc: 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,
	"H. Peter Anvin" <hpa@...or.com>,
	"Kirill A. Shutemov" <kas@...nel.org>,
	Alexander Potapenko <glider@...gle.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Xin Li <xin3.li@...el.com>,
	Sai Praneeth <sai.praneeth.prakhya@...el.com>,
	Jethro Beekman <jethro@...tanix.com>,
	Jarkko Sakkinen <jarkko@...nel.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	Kees Cook <kees@...nel.org>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Yu-cheng Yu <yu-cheng.yu@...el.com>, stable@...r.kernel.org,
	Borislav Petkov <bp@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86: Clear feature bits disabled at compile-time

On Wed, Jul 23, 2025 at 01:46:44PM +0200, Maciej Wieczor-Retman wrote:
> On 2025-07-23 at 11:45:22 +0200, Greg KH wrote:
> >On Wed, Jul 23, 2025 at 11:22:49AM +0200, Maciej Wieczor-Retman wrote:
> >> If some config options are disabled during compile time, they still are
> >> enumerated in macros that use the x86_capability bitmask - cpu_has() or
> >> this_cpu_has().
> >> 
> >> The features are also visible in /proc/cpuinfo even though they are not
> >> enabled - which is contrary to what the documentation states about the
> >> file. Examples of such feature flags are lam, fred, sgx, ibrs_enhanced,
> >> split_lock_detect, user_shstk, avx_vnni and enqcmd.
> >> 
> >> Add a DISABLED_MASK() macro that returns 32 bit chunks of the disabled
> >> feature bits bitmask.
> >> 
> >> Initialize the cpu_caps_cleared and cpu_caps_set arrays with the
> >> contents of the disabled and required bitmasks respectively. Then let
> >> apply_forced_caps() clear/set these feature bits in the x86_capability.
> >> 
> >> Fixes: 6449dcb0cac7 ("x86: CPUID and CR3/CR4 flags for Linear Address Masking")
> >> Fixes: 51c158f7aacc ("x86/cpufeatures: Add the CPU feature bit for FRED")
> >> Fixes: 706d51681d63 ("x86/speculation: Support Enhanced IBRS on future CPUs")
> >> Fixes: e7b6385b01d8 ("x86/cpufeatures: Add Intel SGX hardware bits")
> >> Fixes: 6650cdd9a8cc ("x86/split_lock: Enable split lock detection by kernel")
> >> Fixes: 701fb66d576e ("x86/cpufeatures: Add CPU feature flags for shadow stacks")
> >> Fixes: ff4f82816dff ("x86/cpufeatures: Enumerate ENQCMD and ENQCMDS instructions")
> >
> >That is fricken insane.
> >
> >You are saying to people who backport stuff:
> >	This fixes a commit found in the following kernel releases:
> >		6.4
> >		6.9
> >		3.16.68 4.4.180 4.9.137 4.14.81 4.18.19 4.19
> >		5.11
> >		5.7
> >		6.6
> >		5.10
> >
> >You didn't even sort this in any sane order, how was it generated?
> >
> >What in the world is anyone supposed to do with this?
> >
> >If you were sent a patch with this in it, what would you think?  What
> >could you do with it?
> >
> >Please be reasonable and consider us overworked stable maintainers and
> >give us a chance to get things right.  As it is, this just makes things
> >worse...
> >
> >greg k-h
> 
> Sorry, I certainly didn't want to add you more work.
> 
> I noted down which features are present in the x86_capability bitmask while
> they're not compiled into the kernel. Then I noted down which commits added
> these feature flags. So I suppose the order is from least to most significant
> feature bit, which now I realize doesn't help much in backporting, again sorry.
> 
> Would a more fitting Fixes: commit be the one that changed how the feature flags
> are used? At some point docs started stating to have them set only when features
> are COMPILED & HARDWARE-SUPPORTED.

What would you want to see if you had to do something with a "Fixes:"
line?

thanks,

greg k-h