linux-kernel - Re: [PATCH 4/7] x86/kexec: Disable kexec/kdump on platforms with TDX partial write erratum

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Message-ID: <5d792dc5-ea8e-46d2-8031-44f8e92b0188@suse.com>
Date: Tue, 7 Oct 2025 15:31:42 +0200
From: Jürgen Groß <jgross@...e.com>
To: Dave Hansen <dave.hansen@...el.com>,
 "Reshetova, Elena" <elena.reshetova@...el.com>,
 "Annapurve, Vishal" <vannapurve@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
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 "Chen, Farrah" <farrah.chen@...el.com>, Binbin Wu <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH 4/7] x86/kexec: Disable kexec/kdump on platforms with TDX
 partial write erratum

On 02.10.25 17:06, Dave Hansen wrote:
> On 10/2/25 00:46, Juergen Gross wrote:
>> So lets compare the 2 cases with kdump enabled and disabled in your
>> scenario (crash of the host OS):
>>
>> kdump enabled: No dump can be produced due to the #MC and system is
>> rebooted.
>>
>> kdump disabled: No dump is produced and system is rebooted after crash.
>>> What is the main concern with kdump enabled? I don't see any
>> disadvantage with enabling it, just the advantage that in many cases
>> a dump will be written.
> The disadvantage is that a kernel bug from long ago results in a machine
> check. Machine checks are generally indicative of bad hardware. So the
> disadvantage is that someone mistakes the long ago kernel bug for bad
> hardware.
> 
> There are two ways of looking at this:
> 
> 1. A theoretically fragile kdump is better than no kdump at all. All of
>     the stars would have to align for kdump to _fail_ and we don't think
>     that's going to happen often enough to matter.
> 2. kdump happens after kernel bugs. The machine checks happen because of
>     kernel bugs. It's not a big stretch to think that, at scale, kdump is
>     going to run in to these #MCs on a regular basis.
> 
> Does that capture the two perspectives fairly?

Basically yes.

If we can't come to an agreement that kdump should be allowed in spite of
a potential #MC, maybe we could disable kdump only if TDX guests have been
active on the machine before? Disabling kdump on a distro kernel just because
TDX was enabled but without anyone having used TDX would be quite hard.


Juergen

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