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

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Message-ID: <CAGtprH_nTBdX-VtMQJM4-y8KcB_F4CnafqpDX7ktASwhO0sxAg@mail.gmail.com>
Date: Wed, 1 Oct 2025 10:17:42 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	bp@...en8.de, tglx@...utronix.de, peterz@...radead.org, mingo@...hat.com, 
	hpa@...or.com, thomas.lendacky@....com, x86@...nel.org, kas@...nel.org, 
	rick.p.edgecombe@...el.com, dwmw@...zon.co.uk, kai.huang@...el.com, 
	seanjc@...gle.com, reinette.chatre@...el.com, isaku.yamahata@...el.com, 
	dan.j.williams@...el.com, ashish.kalra@....com, nik.borisov@...e.com, 
	chao.gao@...el.com, sagis@...gle.com, 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 Wed, Oct 1, 2025 at 7:32 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 9/30/25 19:05, Vishal Annapurve wrote:
> ...
> >> Any workarounds are going to be slow and probably imperfect. That's not
> >
> > Do we really need to deploy workarounds that are complex and slow to
> > get kdump working for the majority of the scenarios? Is there any
> > analysis done for the risk with imperfect and simpler workarounds vs
> > benefits of kdump functionality?
> >
> >> a great match for kdump. I'm perfectly happy waiting for fixed hardware
> >> from what I've seen.
> >
> > IIUC SPR/EMR - two CPU generations out there are impacted by this
> > erratum and just disabling kdump functionality IMO is not the best
> > solution here.
>
> That's an eminently reasonable position. But we're speaking in broad
> generalities and I'm unsure what you don't like about the status quo or
> how you'd like to see things change.

Looks like the decision to disable kdump was taken between [1] -> [2].
"The kernel currently doesn't track which page is TDX private memory.
It's not trivial to reset TDX private memory.  For simplicity, this
series simply disables kexec/kdump for such platforms.  This will be
enhanced in the future."

A patch [3] from the series[1], describes the issue as:
"This problem is triggered by "partial" writes where a write transaction
of less than cacheline lands at the memory controller.  The CPU does
these via non-temporal write instructions (like MOVNTI), or through
UC/WC memory mappings.  The issue can also be triggered away from the
CPU by devices doing partial writes via DMA."

And also mentions:
"Also note only the normal kexec needs to worry about this problem, but
not the crash kexec: 1) The kdump kernel only uses the special memory
reserved by the first kernel, and the reserved memory can never be used
by TDX in the first kernel; 2) The /proc/vmcore, which reflects the
first (crashed) kernel's memory, is only for read.  The read will never
"poison" TDX memory thus cause unexpected machine check (only partial
write does)."

What was the scenario that led to disabling kdump support altogether
given the above description?

[1] https://lore.kernel.org/lkml/cover.1727179214.git.kai.huang@intel.com/
[2] https://lore.kernel.org/all/cover.1741778537.git.kai.huang@intel.com/
[3] https://lore.kernel.org/lkml/6960ef6d7ee9398d164bf3997e6009df3e88cb67.1727179214.git.kai.huang@intel.com/

>
> Care to send along a patch representing the "best solution"? That should
> clear things up.
>