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Re: [PATCH] x86/cpu: identify uninitialized CPU data using BAD_APICID
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 4 May 2026 13:31:34 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 04 May 2026 11:31:35 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30.04.2026 13:46, Roger Pau Monne wrote:
> Uninitialized cpu_data[] entries have the apicid field set to BAD_APICID,
> not boot_cpu_data.apicid. Fix the check in cpu_smpboot_free() to use the
> correct condition.
>
> Fixes: 7126b7f806d5 ("x86/CPU: re-work populating of cpu_data[]")
I think this isn't correct, and the issue here is me having overlooked a
dependency between that commit and the one introducing the line which
you change: 8c15d3d18725 ("x86/SMP: guard socket_cpumask[] access in
cpu_smpboot_free()"). Both changes were committed close together, but
they were entirely separate submissions (well over a year apart). Hence
as an individual patch that latter commit was correct, but on top of
7126b7f806d5 it would have needed adjustment. Since 7126b7f806d5 wasn't
backported (while 8c15d3d18725 was), the fix here also shouldn't be
backported to anything earlier than 4.21 (which indeed the Fixes: tag
better expresses).
Jan
> --- a/xen/arch/x86/smpboot.c
> +++ b/xen/arch/x86/smpboot.c
> @@ -961,8 +961,7 @@ static void cpu_smpboot_free(unsigned int cpu, bool
> remove)
> * In that case the socket number cannot be relied upon, but the
> respective
> * socket_cpumask[] slot also wouldn't have been set.
> */
> - if ( c[cpu].apicid != boot_cpu_data.apicid &&
> - cpumask_empty(socket_cpumask[socket]) )
> + if ( c[cpu].apicid != BAD_APICID &&
> cpumask_empty(socket_cpumask[socket]) )
> {
> xfree(socket_cpumask[socket]);
> socket_cpumask[socket] = NULL;
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