linux-kernel - Re: [PATCH v3] x86/boot: skip realmode init code when running as Xen PV guest

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Date:   Thu, 24 Nov 2022 14:30:39 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v3] x86/boot: skip realmode init code when running as Xen
 PV guest

On 24.11.22 14:28, Borislav Petkov wrote:
> On Wed, Nov 23, 2022 at 12:45:23PM +0100, Juergen Gross wrote:
>> When running as a Xen PV guest there is no need for setting up the
>> realmode trampoline, as realmode isn't supported in this environment.
>>
>> Trying to setup the trampoline has been proven to be problematic in
>> some cases, especially when trying to debug early boot problems with
>> Xen requiring to keep the EFI boot-services memory mapped (some
>> firmware variants seem to claim basically all memory below 1M for boot
>> services).
>>
>> Introduce new x86_platform_ops operations for that purpose, which can
>> be set to a nop by the Xen PV specific kernel boot code.
>>
>> Fixes: 084ee1c641a0 ("x86, realmode: Relocator for realmode code")
> 
> This text and Fixes: tag sounds like this needs to go to Linus and
> stable now?

Looking at the date when 084ee1c641a0 went in I don't think it _needs_
to go in now, but I wouldn't complain ...

> 
>> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
>> index 41d7669a97ad..247aca9f8ed1 100644
>> --- a/arch/x86/realmode/init.c
>> +++ b/arch/x86/realmode/init.c
>> @@ -200,14 +200,18 @@ static void __init set_real_mode_permissions(void)
>>   	set_memory_x((unsigned long) text_start, text_size >> PAGE_SHIFT);
>>   }
>>   
>> -static int __init init_real_mode(void)
>> +void __init init_real_mode(void)
>>   {
>>   	if (!real_mode_header)
>>   		panic("Real mode trampoline was not allocated");
>>   
>>   	setup_real_mode();
>>   	set_real_mode_permissions();
>> +}
>>   
>> +static int __init call_init_real_mode(void)
>> +{
>> +	x86_platform.realmode_init();
>>   	return 0;
>>   }
>> -early_initcall(init_real_mode);
>> +early_initcall(call_init_real_mode);
> 
> I'll name that one "do_init_real_mode" as "call init" sounds weird.

Fine with me.

> 
> Otherwise, it is as straightforward as it gets.

Thanks,


Juergen


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