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Re: [PATCH] x86/boot: Fix comment about setting up the real mode stack
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 27 Feb 2025 23:18:42 +0000
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, "Daniel P . Smith" <dpsmith@xxxxxxxxxxxxxxxxxxxx>, Frediano Ziglio <frediano.ziglio@xxxxxxxxx>, Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 27 Feb 2025 23:19:04 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 15/11/2024 9:28 am, Jan Beulich wrote:
> On 14.11.2024 19:22, Andrew Cooper wrote:
>> It may have taken a while, but it occurs to me that the mentioned commit
>> fixed
>> a second problem too.
>>
>> On entering trampoline_boot_cpu_entry(), %esp points at the trampoline stack,
>> but in a 32bit flat segment. It happens to be page aligned.
>>
>> When dropping into 16bit mode, the stack segment operates on %sp, preserving
>> the upper bits. Prior to 1ed76797439e, the top nibble of %sp would depend on
>> where the trampoline was placed in low memory, and only had a 1/16 chance of
>> being 0 and therefore operating on the intended stack.
>>
>> There was a 15/16 chance of using a different page in the trampoline as if it
>> were the stack. Therefore, zeroing %esp was correct, but for more reasons
>> than realised at the time.
> I'm afraid I don't follow this analysis. Said commit replaced clearing of %sp
> by clearing of %esp.
Correct
> That made no difference for anything using the 16-bit
> register.
True, but Xen's 16bit code isn't very relevant to the analysis.
Fujitsu's BIOS is.
> I don't see how the top nibble of %sp could have been non-zero
> prior to that change.
Oh, that's a typo. It should have been the 5th nibble of %esp.
Said nibble depends entirely on where the trampoline is placed in low
memory.
We first enter the trampoline in 32bit flat mode with %esp being the
absolute address of the stack. i.e. it's 0x000yyyyy with a 15/16th's
chance of the 5th nibble being non-zero.
Then we drop down into Real mode (non flat, because the trampoline never
overlaps the IVT at 0). At this point we used to zero %sp which
preserves %esp's 5th nibble.
And in the case that went wrong, INT $0x15 corrupted memory that
happened to be in the Xen image.
Anyway, when I was debugging 11 years ago, I noticed that %esp was
nonzero in its upper half and, despite deciding this was suspicious,
couldn't figure out why and zeroing it all fixed the memory corruption.
I also didn't appreciate that `xor %sp, %sp` was strongly dependent on
the trampoline being at exactly trampoline_start + 64k.
Anyway, given that everyone seemed to be confused, I guess I need to try
rewriting the commit message.
~Andrew
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