linux-kernel - Re: [patch V2 50/58] x86/apic: Provide common init infrastructure

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Date:   Mon, 31 Jul 2023 15:01:22 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Andrew Cooper <andrew.cooper3@...rix.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wei Liu <wei.liu@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Peter Keresztes Schmidt <peter@...esztesschmidt.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [patch V2 50/58] x86/apic: Provide common init infrastructure

On Mon, Jul 31 2023 at 13:31, Juergen Gross wrote:
> On 24.07.23 15:35, Thomas Gleixner wrote:
>>   static void __init xen_apic_check(void)
>>   {
>> -	if (apic == &xen_pv_apic)
>> -		return;
>> -
>> -	pr_info("Switched APIC routing from %s to %s.\n", apic->name,
>> -		xen_pv_apic.name);
>> -	apic = &xen_pv_apic;
>> +	apic_install_driver(&xen_pv_apic);
>>   }
>> +
>>   void __init xen_init_apic(void)
>>   {
>>   	x86_apic_ops.io_apic_read = xen_io_apic_read;
>>   	/* On PV guests the APIC CPUID bit is disabled so none of the
>>   	 * routines end up executing. */
>>   	if (!xen_initial_domain())
>> -		apic = &xen_pv_apic;
>> +		apic_install_driver(&xen_pv_apic);
>
> This is working, but it produces a WARN() splat when booting as an unprivileged
> Xen PV guest from static_call patching (static_call_init() hasn't been called
> yet).

Duh, yes. It's too early.

> The diff below on top is fixing the issue:
>
> diff --git a/arch/x86/xen/apic.c b/arch/x86/xen/apic.c
> index 1838aefc632f..84f24268670b 100644
> --- a/arch/x86/xen/apic.c
> +++ b/arch/x86/xen/apic.c
> @@ -163,14 +163,18 @@ static void __init xen_apic_check(void)
>          apic_install_driver(&xen_pv_apic);
>   }
>
> +void __init xen_apic_install(void)
> +{
> +       /*
> +        * On PV guests the APIC CPUID bit is disabled so none of the
> +        * routines end up executing.
> +        */
> +       apic_install_driver(&xen_pv_apic);
> +}
> +
>   void __init xen_init_apic(void)
>   {
>          x86_apic_ops.io_apic_read = xen_io_apic_read;
> -       /* On PV guests the APIC CPUID bit is disabled so none of the
> -        * routines end up executing. */
> -       if (!xen_initial_domain())
> -               apic_install_driver(&xen_pv_apic);
> -
>          x86_platform.apic_post_init = xen_apic_check;
>   }
>   apic_driver(xen_pv_apic);

I wonder whether this explicit install is actually needed at all.
Shouldn't the driver be installed via the APIC probing mechanism
automagically?

Thanks,

        tglx