From: David Laight <david.laight.linux@gmail.com>
To: Maciej Wieczor-Retman <m.wieczorretman@pm.me>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
tglx@kernel.org, peterz@infradead.org, xin@zytor.com,
maciej.wieczor-retman@intel.com, babu.moger@amd.com,
chang.seok.bae@intel.com, sohil.mehta@intel.com,
dave.hansen@linux.intel.com, jpoimboe@kernel.org,
elena.reshetova@intel.com, ak@linux.intel.com,
darwi@linutronix.de, bp@alien8.de, mingo@redhat.com,
x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v11 3/4] x86/cpu: Do a sanity check on required feature bits
Date: Mon, 30 Mar 2026 22:24:24 +0100 [thread overview]
Message-ID: <20260330222424.39948b9c@pumpkin> (raw)
In-Reply-To: <acpIvIYg_6goTRIW@wieczorr-mobl1.localdomain>
On Mon, 30 Mar 2026 10:09:47 +0000
Maciej Wieczor-Retman <m.wieczorretman@pm.me> 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
next prev parent reply other threads:[~2026-03-30 21:24 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-20 12:50 [PATCH v11 0/4] x86: Capability bits fix and required bits sanity check Maciej Wieczor-Retman
2026-03-20 12:50 ` [PATCH v11 1/4] x86/cpu: Clear feature bits disabled at compile-time Maciej Wieczor-Retman
2026-03-20 12:50 ` [PATCH v11 2/4] x86/cpu: Check if feature string is non-zero Maciej Wieczor-Retman
2026-03-23 14:24 ` Borislav Petkov
2026-03-23 15:52 ` Maciej Wieczor-Retman
2026-03-23 16:23 ` Borislav Petkov
2026-03-23 16:58 ` Maciej Wieczor-Retman
2026-03-23 17:51 ` Borislav Petkov
2026-03-23 18:11 ` Maciej Wieczor-Retman
2026-03-23 18:15 ` H. Peter Anvin
2026-03-20 12:50 ` [PATCH v11 3/4] x86/cpu: Do a sanity check on required feature bits Maciej Wieczor-Retman
2026-03-21 0:31 ` Pawan Gupta
2026-03-21 5:58 ` Maciej Wieczór-Retman
2026-03-23 18:16 ` Pawan Gupta
2026-03-23 18:33 ` Maciej Wieczor-Retman
2026-03-26 18:36 ` Maciej Wieczor-Retman
2026-03-26 19:04 ` Pawan Gupta
2026-03-26 19:11 ` Maciej Wieczor-Retman
2026-03-28 1:52 ` H. Peter Anvin
2026-03-28 2:01 ` H. Peter Anvin
2026-03-30 9:47 ` Maciej Wieczor-Retman
2026-03-30 10:09 ` Maciej Wieczor-Retman
2026-03-30 16:01 ` Pawan Gupta
2026-03-30 21:24 ` David Laight [this message]
2026-03-31 8:12 ` Maciej Wieczor-Retman
2026-03-31 13:29 ` Maciej Wieczor-Retman
2026-03-23 16:31 ` Borislav Petkov
2026-03-23 17:05 ` Maciej Wieczor-Retman
2026-03-23 17:55 ` Borislav Petkov
2026-03-23 18:43 ` H. Peter Anvin
2026-03-23 18:43 ` H. Peter Anvin
2026-03-23 19:19 ` Borislav Petkov
2026-03-23 20:24 ` H. Peter Anvin
2026-03-23 20:58 ` Borislav Petkov
2026-03-23 21:40 ` H. Peter Anvin
2026-03-23 21:50 ` Borislav Petkov
2026-03-23 21:56 ` Borislav Petkov
2026-03-23 22:03 ` H. Peter Anvin
2026-03-23 22:09 ` Borislav Petkov
2026-03-24 1:16 ` H. Peter Anvin
2026-03-20 12:50 ` [PATCH v11 4/4] x86/cpu: Clear feature bits whose dependencies were cleared Maciej Wieczor-Retman
2026-03-23 16:35 ` Borislav Petkov
2026-03-23 17:23 ` Maciej Wieczor-Retman
2026-03-23 17:59 ` Borislav Petkov
2026-03-23 18:18 ` Maciej Wieczor-Retman
2026-03-23 18:57 ` H. Peter Anvin
2026-03-23 19:30 ` Borislav Petkov
2026-03-25 9:33 ` Maciej Wieczor-Retman
2026-03-23 19:33 ` Ahmed S. Darwish
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