linux-kernel - Re: [RFC PATCH v5 02/45] KVM: x86/mmu: Update iter->old

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Message-ID: <fbaeb0d2f4658efd4c7bb61ac0ba2919c8226a36.camel@intel.com>
Date: Thu, 29 Jan 2026 22:10:41 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>, "x86@...nel.org"
	<x86@...nel.org>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "kas@...nel.org" <kas@...nel.org>,
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CC: "Huang, Kai" <kai.huang@...el.com>, "ackerleytng@...gle.com"
	<ackerleytng@...gle.com>, "sagis@...gle.com" <sagis@...gle.com>, "Annapurve,
 Vishal" <vannapurve@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Zhao, Yan Y" <yan.y.zhao@...el.com>, "Li,
 Xiaoyao" <xiaoyao.li@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
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Subject: Re: [RFC PATCH v5 02/45] KVM: x86/mmu: Update iter->old_spte if
 cmpxchg64 on mirror SPTE "fails"

On Wed, 2026-01-28 at 17:14 -0800, Sean Christopherson wrote:
> Pass a pointer to iter->old_spte, not simply its value, when setting an
> external SPTE in __tdp_mmu_set_spte_atomic(), so that the iterator's value
> will be updated if the cmpxchg64 to freeze the mirror SPTE fails.
> 

Might be being dense here, but is the bug that if cmpxchg64 *succeeds* and
set_external_spte() fails? Then old_spte is not updated and the local retry will
expect the wrong old_spte.

>   The bug
> is currently benign as TDX is mutualy exclusive with all paths that do
> "local" retry", e.g. clear_dirty_gfn_range() and wrprot_gfn_range().