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

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Date:   Mon, 31 Jul 2023 15:10:21 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        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 31.07.23 15:01, Thomas Gleixner wrote:
> 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?

Only in case x86_init.irq.intr_mode_init is set appropriately. Today it is
a nop for Xen PV, but that can be changed. I'll have a look.


Juergen

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