Re: [PATCH v11 3/4] x86/cpu: Do a sanity check on required feature bits

[David Laight]

On Mon, 30 Mar 2026 10:09:47 +0000
Maciej Wieczor-Retman <m.wieczorretman@xxxxx> wrote:

> On 2026-03-27 at 18:52:30 -0700, H. Peter Anvin wrote:
...
> >One thing that matters here is that these bitmaps are *already* accessed using
> >bitop operations. Therefore, if this is a problem *here*, then it is a problem
> >*everywhere*.   
> 
> I think for example the set_bit()/clear_bit() bitops are not problematic while
> for_each_set_bit() is, specfically used in this context. Most operations seem to
> not affect or not be affected by the potential unaligned 32-bit.

Oh they are...
Look up 'split lock'.

You must not cast int[] to long[] for the 'bit' functions.

Basically if the 'long' gets split over a cache-line boundary the cpu
has to do an 'old fashioned' lock of the inter-cpu bus in order to
perform the locked memory accesses.
That is both slow and kills the rest of the machine.

	David