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

From: Xin Li
Date: Wed Jul 23 2025 - 11:53:30 EST

On 7/23/2025 6:03 AM, Maciej Wieczor-Retman wrote:
On 2025-07-23 at 13:57:34 +0200, Greg KH wrote:
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?

I suppose I'd want to see a Fixes:commit in a place that hasn't seen too many
changes. So the backport process doesn't hit too many infrastructure changes
since that makes things more tricky.

And I guess it would be great if the Fixes:commit pointed at some obvious error
that happened - like a place that could dereference a NULL pointer for example.

But I thought Fixes: was supposed to mark the origin point of some error the
patch is fixing?

In this case a documentation patch [1] changed how feature flags are supposed to
behave. But these flags were added in various points in the past. So what should
Fixes: point at then?

But anyway writing this now I get the feeling that [1] would be a better point
to mark for the "Fixes:" line.

[1] ea4e3bef4c94 ("Documentation/x86: Add documentation for /proc/cpuinfo feature flags")


We need to investigate the root cause of this bug; and this seems like a
regression caused by my patch set that automatically generates CPU feature masks.

Just further debugging is needed to identify the culprit.

Thanks!
Xin