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Re: [PATCH v4 2/4] x86/boot: Refactor BIOS/PVH start
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 26 Sep 2024 08:50:26 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Frediano Ziglio <frediano.ziglio@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 26 Sep 2024 06:50:29 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 25.09.2024 21:33, Andrew Cooper wrote:
> On 25/09/2024 7:00 am, Frediano Ziglio wrote:
>> @@ -449,62 +458,40 @@ __pvh_start:
>> mov %ecx, %es
>> mov %ecx, %ss
>>
>> - /* Skip bootloader setup and bios setup, go straight to trampoline
>> */
>> - movb $1, sym_esi(pvh_boot)
>> - movb $1, sym_esi(skip_realmode)
>> + /* Load null selector to unused segment registers. */
>> + xor %ecx, %ecx
>> + mov %ecx, %fs
>> + mov %ecx, %gs
>
> Honestly, the more I look at this, the more bizarre it is.
>
> We should just set up %fs/gs like we do %ds/es, which in this case is
> simply to drop the comment and the xor.
What's bizarre here? As long as we don't use %fs/%gs, it doesn't matter
much what we set them to. So yes, they may be set to what %ds etc are set
to, but they may as well be marked unusable. Documentation-wise that's
cleaner imo, as down the road - when a need to use one arises - it then
won't require auditing of all code to figure where the register(s) is(are)
actually used (just to find: nowhere). Even if a comment to this effect
was left here, I for one wouldn't trust it in a couple of years time, but
rather fear it went stale.
Jan
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