Re: [PATCH v4 24/24] [NOT-FOR-REVIEW] x86/virt/seamldr: Save and restore current VMCS

[Huang, Kai]

> 
> But I agree that following the X86_BUG_TDX_PW_MCE is better in consistency
> and extensibility. So, here is the refined patch:
> 
> 
> From 46e89a50803d6568eb60bd8ec866ac3fd9f6e6da Mon Sep 17 00:00:00 2001
> From: Chao Gao <chao.gao@xxxxxxxxx>
> Date: Tue, 10 Mar 2026 18:49:41 -0700
> Subject: [PATCH] coco/tdx-host: Don't expose P-SEAMLDR features on CPUs with
>  erratum
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> Some TDX-capable CPUs have an erratum, as documented in Intel® Trust
> Domain CPU Architectural Extensions (May 2021 edition) Chapter 2.3:
> 
>   SEAMRET from the P-SEAMLDR clears the current VMCS structure pointed
>   to by the current-VMCS pointer. A VMM that invokes the P-SEAMLDR using
>   SEAMCALL must reload the current-VMCS, if required, using the VMPTRLD
>   instruction.
> 
> Clearing the current VMCS behind KVM's back will break KVM.
> 
> This erratum is not present when IA32_VMX_BASIC[60] is set. Add a CPU
> bug bit for this erratum and refuse to expose P-SEAMLDR features (e.g.,
> TDX module updates) on affected CPUs. Also, emit a message to clarify
> why P-SEAMLDR features are disabled for affected CPUs.

The user can actually see this new bug flag in /proc/cpuinfo, so the error
message may not be mandatory.  It's fine to me anyway, so will leave to
others.

> 
> == Alternatives ==
> Two workarounds were considered but both were rejected:
> 
> 1. Save/restore the current VMCS around P-SEAMLDR calls. This produces ugly
>    assembly code [1] and doesn't play well with #MCE or #NMI if they
>    need to use the current VMCS.
> 
> 2. Move KVM's VMCS tracking logic to the TDX core code, which would break
>    the boundary between KVM and the TDX core code [2].
> 
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> Link: https://lore.kernel.org/kvm/fedb3192-e68c-423c-93b2-a4dc2f964148@xxxxxxxxx/ # [1]
> Link: https://lore.kernel.org/kvm/aYIXFmT-676oN6j0@xxxxxxxxxx/ # [2]

LGTM:

Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>

One nit below:


[...]

> +#define X86_BUG_SEAMRET_INVD_VMCS	X86_BUG( 1*32+11) /* "seamret_invd_vmcs" SEAMRET may clear the current VMCS */

"may clear" -> "clears" ?