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From: "Eric W. Biederman" <ebiederm@xmission.com>
To: Ingo Molnar <mingo@kernel.org>
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Date: Sat, 30 Mar 2024 22:55:53 -0500
In-Reply-To: <ZgbCfVPvWroCnGE4@gmail.com> (Ingo Molnar's message of "Fri, 29
	Mar 2024 14:30:37 +0100")
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Subject: Re: [PATCH v4] x86/mm/ident_map: On UV systems, use gbpages only
 where full GB page should be mapped.

Ingo Molnar <mingo@kernel.org> writes:

> * Pavin Joseph <me@pavinjoseph.com> wrote:
>
>> On 3/29/24 13:45, Ingo Molnar wrote:
>> > Just to clarify, we have the following 3 upstream (and soon to be upstream) versions:
>> > 
>> >   v1: pre-d794734c9bbf kernels
>> >   v2: d794734c9bbf x86/mm/ident_map: Use gbpages only where full GB page should be mapped.
>> >   v3: c567f2948f57 Revert "x86/mm/ident_map: Use gbpages only where full GB page should be mapped."
>> > 
>> > Where v1 and v3 ought to be the same in behavior.
>> > 
>> > So how does the failure matrix look like on your systems? Is my
>> > understanding accurate:
>
>> Slight correction:
>> 
>>    regular boot  | regular kexec | nogbpages boot | nogbpages kexec boot
>> -----------------|---------------|----------------|------------------
>> v1:       OK     | OK            | OK             | FAIL
>> v2:       OK     | FAIL          | OK             | FAIL
>
> Thanks!
>
> So the question is now: does anyone have a theory about in what fashion 
> the kexec nogbpages bootup differs from the regular nogbpages bootup to 
> break on your system?
>
> I'd have expected the described root cause of the firmware not properly 
> enumerating all memory areas that need to be mapped to cause trouble on 
> regular, non-kexec nogbpages bootups too. What makes the kexec bootup 
> special to trigger this crash?

My blind hunch would be something in the first 1MiB being different.
The first 1MiB is where all of the historical stuff is and where
I have seen historical memory maps be less than perfectly accurate.

Changing what is mapped being the difference between success and failure
sounds like some place that is dark and hard to debug a page fault is
being triggered and that in turn becoming a triple fault.

Paving Joseph is there any chance you can provide your memory map?
Perhaps just cat /proc/iomem?

If I have something to go one other than works/doesn't work I can
probably say something intelligent.

Eric