linux-kernel - Re: [RFC PATCH 00/20] TD-Preserving updates

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Message-ID: <aHEaXYmeolKNCqgk@google.com>
Date: Fri, 11 Jul 2025 07:06:21 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: linux-coco@...ts.linux.dev, x86@...nel.org, kvm@...r.kernel.org, 
	paulmck@...nel.org, pbonzini@...hat.com, eddie.dong@...el.com, 
	kirill.shutemov@...el.com, dave.hansen@...el.com, dan.j.williams@...el.com, 
	kai.huang@...el.com, isaku.yamahata@...el.com, elena.reshetova@...el.com, 
	rick.p.edgecombe@...el.com, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Ingo Molnar <mingo@...hat.com>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 00/20] TD-Preserving updates

On Fri, Jul 11, 2025, Chao Gao wrote:
> >2. P-SEAMLDR seamcalls (specificially SEAMRET from P-SEAMLDR) clear current
> >   VMCS pointers, which may disrupt KVM. To prevent VMX instructions in IRQ
> >   context from encountering NULL current-VMCS pointers, P-SEAMLDR
> >   seamcalls are called with IRQ disabled. I'm uncertain if NMIs could
> >   cause a problem, but I believe they won't. See more information in patch 3.

NMIs shouldn't be a problem.  KVM does access the current VMCS in NMI context
(to do VMREAD(GUEST_RIP) in response to a perf NMI), but only when KVM knows the
NMI occurred in KVM's run loop.  So in effect, only in KVM_RUN context, which I
gotta image is mutually exclusive with tdx_fw_write().

It'd be nice if we could make the P-SEAMLDR calls completely NMI safe, but
practically speaking, if KVM (or any other hypervisor) is playing with the VMCS
in arbitrary NMI handlers, then we've probably got bigger issues.