linux-kernel - Re: [RFC PATCH 3/5] x86/kexec: Disable kexec/kdump on platforms with TDX partial write erratum

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Message-ID: <da02e1d5a489526770ec737eac41237226cdb5cd.camel@intel.com>
Date: Thu, 13 Mar 2025 22:32:55 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "tglx@...utronix.de" <tglx@...utronix.de>, "peterz@...radead.org"
	<peterz@...radead.org>, "mingo@...hat.com" <mingo@...hat.com>, "Hansen, Dave"
	<dave.hansen@...el.com>, "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"bp@...en8.de" <bp@...en8.de>, "kirill.shutemov@...ux.intel.com"
	<kirill.shutemov@...ux.intel.com>
CC: "nik.borisov@...e.com" <nik.borisov@...e.com>, "bhe@...hat.com"
	<bhe@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>, "x86@...nel.org"
	<x86@...nel.org>, "sagis@...gle.com" <sagis@...gle.com>, "hpa@...or.com"
	<hpa@...or.com>, "Chatre, Reinette" <reinette.chatre@...el.com>, "Williams,
 Dan J" <dan.j.williams@...el.com>, "thomas.lendacky@....com"
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	"pbonzini@...hat.com" <pbonzini@...hat.com>, "Yamahata, Isaku"
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	<linux-kernel@...r.kernel.org>, "dwmw@...zon.co.uk" <dwmw@...zon.co.uk>
Subject: Re: [RFC PATCH 3/5] x86/kexec: Disable kexec/kdump on platforms with
 TDX partial write erratum

On Thu, 2025-03-13 at 17:18 +0000, Edgecombe, Rick P wrote:
> On Thu, 2025-03-13 at 00:57 +0000, Huang, Kai wrote:
> > > Continuing an internal discussion... A problem with the plan to more simply
> > > start support for kexec by not supporting the errata platforms is that when
> > > these platforms configure TDX they will lose kexec by default.
> > > 
> > > Probably a better default for a lot of kernels would be to have kexec work
> > > by
> > > default, and require opt-in to use TDX (and lose kexec). One idea was a
> > > kernel
> > > parameter be required to enable TDX on those platforms. But then we are
> > > starting
> > > to add complexity to avoid other complexity (the errata platform kexec
> > > support).
> > > 
> > > Still, it may be a net win on complexity.
> > 
> > We can add a kernel parameter 'tdx_host={on|off}' and skip all TDX code (thus
> > no
> > erratum detection) when it is off.  I suppose it will be useful in general
> > anyway even w/o the context of kexec.
> 
> What exactly are you thinking? Add a tdx_host parameter, but what is the default
> behavior? When tdx_host=on with the errata, kexec must still be disabled, right?
> Better to return an error, than proceed and crash.

The default behaviour is tdx_host=off in order to not disrupt kexec/kdump
behaviour on the TDX platforms with erratum.  The distros will be able to ship
kernels with both CONFIG_KEXEC_CORE and CONFIG_INTEL_TDX_HOST on, and no visible
impact to the user who doesn't care about TDX.

If the user is interested in TDX, tdx_host=on must be set in the kernel command
line, but in this case user is expected to know kexec/kdump can only work
normally if the TDX platform doesn't have the erratum -- kexec/kdump are
disabled if the platform has the erratum.