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Re: [PATCH 1/4] x86/kexec: Stop hooking NMIs with trap_nop()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 24 Mar 2026 12:52:49 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 24 Mar 2026 11:52:59 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 19.03.2026 13:25, Andrew Cooper wrote:
> When FRED is active, it is not possible to hook NMIs like this.
>
> NMI hooking in the crash path has undergone several revisions since its
> introduction. Notably since commit e7f147bf4ac7 ("x86/crash: Drop manual
> hooking of exception_table[]") we use the regular nmi_callback()
> infrastructure.
>
> Instead of asserting that we don't enter do_nmi_crash() on the crashing CPU,
> tolerate it and return early. It's a marginally longer codepath but behaves
> the same and is compatible with FRED.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> The other use of hooking the NMI handler like this is in play_dead() and
> introduced by commit 73cb1383bf8d ("x86/idle: re-arrange dead-idle
> handling"). It's unsafe, and the commit even mentions so for #MC.
Why is wiring to trap_nop() unsafe? There is a per-CPU access when
PERF_COUNTERS=y, but otherwise?
For FRED, shouldn't do_nmi() then gain a similar early exit for offlined
CPUs, replacing the IDT editing?
> On x86, we simply cannot free the per-cpu block for any CPU that hasn't been
> put back into the wait-for-SIPI state.
Please remind me, is there a reason we can't put CPUs we have offlined (not
parked) into that state?
Jan
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