linux-kernel - Re: [PATCH v6 24/29] context_tracking,x86: Defer kernel text patching IPIs

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Message-ID: <aQDX6hHV3b1-_wuF@localhost.localdomain>
Date: Tue, 28 Oct 2025 15:49:14 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Valentin Schneider <vschneid@...hat.com>
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Subject: Re: [PATCH v6 24/29] context_tracking,x86: Defer kernel text
 patching IPIs

Le Fri, Oct 10, 2025 at 05:38:34PM +0200, Valentin Schneider a écrit :
> text_poke_bp_batch() sends IPIs to all online CPUs to synchronize
> them vs the newly patched instruction. CPUs that are executing in userspace
> do not need this synchronization to happen immediately, and this is
> actually harmful interference for NOHZ_FULL CPUs.
> 
> As the synchronization IPIs are sent using a blocking call, returning from
> text_poke_bp_batch() implies all CPUs will observe the patched
> instruction(s), and this should be preserved even if the IPI is deferred.
> In other words, to safely defer this synchronization, any kernel
> instruction leading to the execution of the deferred instruction
> sync (ct_work_flush()) must *not* be mutable (patchable) at runtime.
> 
> This means we must pay attention to mutable instructions in the early entry
> code:
> - alternatives
> - static keys
> - static calls
> - all sorts of probes (kprobes/ftrace/bpf/???)
> 
> The early entry code leading to ct_work_flush() is noinstr, which gets rid
> of the probes.
> 
> Alternatives are safe, because it's boot-time patching (before SMP is
> even brought up) which is before any IPI deferral can happen.
> 
> This leaves us with static keys and static calls.
> 
> Any static key used in early entry code should be only forever-enabled at
> boot time, IOW __ro_after_init (pretty much like alternatives). Exceptions
> are explicitly marked as allowed in .noinstr and will always generate an
> IPI when flipped.
> 
> The same applies to static calls - they should be only updated at boot
> time, or manually marked as an exception.
> 
> Objtool is now able to point at static keys/calls that don't respect this,
> and all static keys/calls used in early entry code have now been verified
> as behaving appropriately.
> 
> Leverage the new context_tracking infrastructure to defer sync_core() IPIs
> to a target CPU's next kernel entry.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Nicolas Saenz Julienne <nsaenzju@...hat.com>
> Signed-off-by: Valentin Schneider <vschneid@...hat.com>

Acked-by: Frederic Weisbecker <frederic@...nel.org>

-- 
Frederic Weisbecker
SUSE Labs