linux-kernel - Re: [PATCH] i386: always clear bss

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Date:	Fri, 04 May 2007 07:57:52 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] i386: always clear bss

Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@...p.org> writes:
>
>   
>> When the paravirt dispatcher gets run immediately on entry to
>> startup_32, the bss isn't cleared.  This happens to work if the
>> hypervisor's domain builder loaded the complete kernel image and
>> cleared the bss for us, but this may not always be true (for example,
>> if we're running out of a decompressed bzImage).
>>
>> Change head.S so that it unconditionally clears the bss before doing
>> the paravirt dispatch or continuing on to normal native boot.
>>
>> There are a couple of points to note:
>>  - We can't, in general, load the segment registers before paravirt
>>    dispatch, because we could be running with a non-standard gdt and
>>    segment selectors.  In practice though, all code which ends up
>>    jumping into startup_32 will have already set the segment registers
>>    up to sane values, so we don't need to do it again.
>>  - Paging may or may not be enabled, and if enabled we may or may not
>>    be mapped to the proper kernel virtual address.  To deal with this,
>>    we compare the kernel's linked address with where we're actually
>>    running, and use that to offset the bss pointer.
>>     

BTW, I should have marked this as an RFC comment, rather than an actual
submission.  We don't need it for .22.

> NAK.  
>
> Skipping the segment register load is likely fine.
> Supporting V!=P at startup_32 is not.
>   

Why?

> Assuming that we have a stack at startup_32 is not.
>
> If you want to figure out where the kernel is loaded you can do
> (from arch/i386/boot/head.S)
>   

Yes, that's more or less the same code, aside from using 0x40(%esi) as a
stack.  Would that be OK here?

    J
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