linux-kernel - Re: [PATCH v2.1 2/5] x86/virt/tdx: Mark memory cache state incoherent when making SEAMCALL

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Message-ID: <66f5b7f0-8109-4c10-80ef-783ba8bccae6@intel.com>
Date: Wed, 14 May 2025 08:52:20 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Kai Huang <kai.huang@...el.com>, bp@...en8.de, tglx@...utronix.de,
 peterz@...radead.org, mingo@...hat.com
Cc: kirill.shutemov@...ux.intel.com, hpa@...or.com, x86@...nel.org,
 linux-kernel@...r.kernel.org, pbonzini@...hat.com, seanjc@...gle.com,
 rick.p.edgecombe@...el.com, reinette.chatre@...el.com,
 isaku.yamahata@...el.com, dan.j.williams@...el.com, thomas.lendacky@....com,
 ashish.kalra@....com, nik.borisov@...e.com, sagis@...gle.com
Subject: Re: [PATCH v2.1 2/5] x86/virt/tdx: Mark memory cache state incoherent
 when making SEAMCALL

On 5/14/25 03:10, Kai Huang wrote:
> Turn on that boolean when the kernel does SEAMCALL so that kexec can
> correctly flush cache.  Note not all SEAMCALL leaf functions generate
> dirty cachelines of TDX private memory, but for simplicity, just treat
> all of them do.

It's not just for simplicity.

There's no contract in place for when the TDX module will dirty memory
or not. A call that is "clean" today might dirty memory tomorrow.

The _only_ thing we know is that SEAMCALLs can dirty cachelines. We
don't know when or how they do it. This blurb makes it sound like it's
possible to optimize this. It's not.