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

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Message-ID: <ac4d337f508eef06f354b60e4f9baff2eb727fde.camel@intel.com>
Date: Thu, 29 Jan 2026 22:48:12 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "Huang, Kai" <kai.huang@...el.com>, "Li,
 Xiaoyao" <xiaoyao.li@...el.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kas@...nel.org" <kas@...nel.org>, "mingo@...hat.com" <mingo@...hat.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>, "ackerleytng@...gle.com"
	<ackerleytng@...gle.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
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	"bp@...en8.de" <bp@...en8.de>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH v5 02/45] KVM: x86/mmu: Update iter->old_spte if
 cmpxchg64 on mirror SPTE "fails"

On Thu, 2026-01-29 at 14:23 -0800, Sean Christopherson wrote:
> No, the bug is if the cmpxchg64 fails.  On failure, the current mismatching value
> is stored in the "old" param.  KVM relies on the iter->old_spte holding the
> current value when restarting an operation without re-reading the SPTE from memory.

Ah, I see. Sorry. Just went and refreshed up on the difference between
cmpxchg64() and try_cmpxchg64(). I see now that the log is accurate since it
refers to the behavior of the instruction, but specifying try_cmpxchg64() might
be a little clearer since cmpxchg() doesn't automatically update the 'old'
passed in. In either case:

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>