Re: [PATCH 8/8] x86/EFI: don't have an overly large image size

From: Jan Beulich <jbeulich@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH 8/8] x86/EFI: don't have an overly large image size
Date: Wed, 21 Apr 2021 15:15:51 +0200	[thread overview]
Message-ID: <5b9af9cc-fdd6-aa16-f902-0c5836bb2cbc@suse.com> (raw)
In-Reply-To: <YIAKIx0JuYDzEKqK@Air-de-Roger>

On 21.04.2021 13:18, Roger Pau Monné wrote:
> On Thu, Apr 01, 2021 at 11:47:35AM +0200, Jan Beulich wrote:
>> While without debug info the difference is benign (so far), since we pad
>> the image to 16Mb anyway, forcing the .reloc section to a 2Mb boundary
>> causes subsequent .debug_* sections to go farther beyond 16Mb than
>> needed. There's no reason to advance . for establishing __2M_rwdata_end,
>> as all data past _end is of no interest at runtime anymore anyway.
> 
> So you just expand the load size.

Shrink. Or maybe I'm misunderstanding you.

>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> 
> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks.

>> ---
>> This makes more explicit a possible latent problem with the ELF image:
>> It ends at _end, not __2M_rwdata_end (advancing . as was done here does
>> not have the effect of increasing the image size). Interestingly the
>> conversion xen-syms => xen rounds up the program header specified size
>> suitably, as per the comment "Do not use p_memsz: it does not include
>> BSS alignment padding" in mkelf32.c. I do think this would instead want
>> taking care of in the linker script. Commit 7a95e0a2c572 ("x86: properly
>> calculate xen ELF end of image address") clearly only hacked an existing
>> hack rather than addressing the root cause. Thoughts?
> 
> We should likely define _end after __2M_rwdata_end to account for this
> padding?

I don't think this would help - we'd need to arrange for the image size
to cover that extra padding. Like advancing . doesn't grow the image
size, I also don't think placing _end later would do.

Jan