linux-kernel - Re: [PATCH v2 1/3] x86/xen: use clear

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Date:   Tue, 28 Jun 2022 14:03:15 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] x86/xen: use clear_bss() for Xen PV guests

On 23.06.22 11:51, Jan Beulich wrote:
> On 23.06.2022 11:46, Juergen Gross wrote:
>> --- a/arch/x86/xen/enlighten_pv.c
>> +++ b/arch/x86/xen/enlighten_pv.c
>> @@ -1183,15 +1183,19 @@ static void __init xen_domu_set_legacy_features(void)
>>   extern void early_xen_iret_patch(void);
>>   
>>   /* First C function to be called on Xen boot */
>> -asmlinkage __visible void __init xen_start_kernel(void)
>> +asmlinkage __visible void __init xen_start_kernel(struct start_info *si)
>>   {
>>   	struct physdev_set_iopl set_iopl;
>>   	unsigned long initrd_start = 0;
>>   	int rc;
>>   
>> -	if (!xen_start_info)
>> +	if (!si)
>>   		return;
>>   
>> +	clear_bss();
> 
> As per subsequent observation, this shouldn't really be needed: The
> hypervisor (or tool stack for DomU-s) already does so. While I guess
> we want to keep it to be on the safe side, maybe worth a comment?

Are you sure all possible boot loaders are clearing alloc-only sections?

I'd rather not count on e.g. grub doing this in all cases.


Juergen

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