linux-kernel - Re: [PATCH] x86/xen/smp_pv: Register the boot CPU APIC properly

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Date: Thu, 2 May 2024 16:42:02 +0200
From: Jürgen Groß <jgross@...e.com>
To: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
 x86@...nel.org
Cc: Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH] x86/xen/smp_pv: Register the boot CPU APIC properly

On 02.05.24 16:39, Thomas Gleixner wrote:
> The topology core expects the boot APIC to be registered from earhy APIC
> detection first and then again when the firmware tables are evaluated. This
> is used for detecting the real BSP CPU on a kexec kernel.
> 
> The recent conversion of XEN/PV to register fake APIC IDs failed to
> register the boot CPU APIC correctly as it only registers it once. This
> causes the BSP detection mechanism to trigger wrongly:
> 
>     CPU topo: Boot CPU APIC ID not the first enumerated APIC ID: 0 > 1
> 
> Additionally this results in one CPU being ignored.
> 
> Register the boot CPU APIC twice so that the XEN/PV fake enumeration
> behaves like real firmware.
> 
> Reported-by: Juergen Gross <jgross@...e.com>
> Fixes: e75307023466 ("x86/xen/smp_pv: Register fake APICs")
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Tested-by: Juergen Gross <jgross@...e.com>

Reviewed-by: Juergen Gross <jgross@...e.com>

Thanks for the patch, I'll take it via the Xen tree.


Juergen

> ---
>   arch/x86/xen/smp_pv.c |    4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- 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(voi
>   	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 */