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From: Thomas Gleixner <tglx@linutronix.de>
To: Juergen Gross <jgross@suse.com>, linux-kernel@vger.kernel.org,
 x86@kernel.org
Cc: Juergen Gross <jgross@suse.com>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
 "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 1/2] x86/cpu: fix BSP detection when running as Xen PV
 guest
In-Reply-To: <20240405123434.24822-2-jgross@suse.com>
References: <20240405123434.24822-1-jgross@suse.com>
 <20240405123434.24822-2-jgross@suse.com>
Date: Tue, 30 Apr 2024 18:13:58 +0200
Message-ID: <87ttjisu9l.ffs@tglx>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain

On Fri, Apr 05 2024 at 14:34, Juergen Gross wrote:
> When booting as a Xen PV guest the boot processor isn't detected
> correctly and the following message is shown:
>
>   CPU topo: Boot CPU APIC ID not the first enumerated APIC ID: 0 > 1
>
> Additionally this results in one CPU being ignored.
>
> Fix that by calling the BSP detection logic when registering the boot
> CPU's APIC, too.
>
> Fixes: 5c5682b9f87a ("x86/cpu: Detect real BSP on crash kernels")
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  arch/x86/kernel/cpu/topology.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
> index aaca8d235dc2..23c3db5e6396 100644
> --- a/arch/x86/kernel/cpu/topology.c
> +++ b/arch/x86/kernel/cpu/topology.c
> @@ -255,7 +255,7 @@ void __init topology_register_boot_apic(u32 apic_id)
>  	WARN_ON_ONCE(topo_info.boot_cpu_apic_id != BAD_APICID);
>  
>  	topo_info.boot_cpu_apic_id = apic_id;
> -	topo_register_apic(apic_id, CPU_ACPIID_INVALID, true);
> +	topology_register_apic(apic_id, CPU_ACPIID_INVALID, true);

No. This does not fix anything at all. It just papers over the
underlying problem.

Thanks,

        tglx
---
diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 27d1a5b7f571..ac41d83b38d3 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -154,9 +154,9 @@ static void __init xen_pv_smp_config(void)
 	u32 apicid = 0;
 	int i;
 
-	topology_register_boot_apic(apicid++);
+	topology_register_boot_apic(apicid);
 
-	for (i = 1; i < nr_cpu_ids; i++)
+	for (i = 0; i < nr_cpu_ids; i++)
 		topology_register_apic(apicid++, CPU_ACPIID_INVALID, true);
 
 	/* Pretend to be a proper enumerated system */