From: Sean Christopherson <seanjc@google.com>
To: David Kaplan <David.Kaplan@amd.com>
Cc: kvm list <kvm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Andrew Cooper <andrew.cooper3@citrix.com>,
Thomas Lendacky <Thomas.Lendacky@amd.com>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: Bug with nested PAUSE intercept on SVM
Date: Tue, 7 Apr 2026 11:24:54 -0700 [thread overview]
Message-ID: <adVL9nqJhJwvu1QO@google.com> (raw)
In-Reply-To: <DS7PR12MB820101CABFA34E85442EA256945AA@DS7PR12MB8201.namprd12.prod.outlook.com>
On Tue, Apr 07, 2026, David Kaplan wrote:
> Hi,
>
> On AMD SVM when the L1 guest is trying to intercept every PAUSE instruction
> in an L2 guest, the PAUSE intercept sometimes fails to fire. I have a theory
> on the source of the bug and also included a short reproducer below.
>
> In this scenario, L1 has created a guest with the pause count and threshold
> set to 0, and the PAUSE intercept bit set. I *think* the bug is that if the
> vCPU gets scheduled out on L0 while we're in the L2 guest, then upon resuming
> the vCPU KVM calls shrink_ple_window() which doesn't appear to take into
> account the fact that svm->vmcb might be for the L2 guest and not the L1. As
> a result, it looks like it sets the pause count to the default (3000) causing
> many PAUSE instructions in L2 to not be intercepted.
It's probably even simpler than that: KVM is completely broken.
https://lore.kernel.org/all/20250131010601.469904-1-seanjc@google.com
Paolo, can I finally apply that patch? I brought it up in PUCK a while back,
and IIRC you were resistant to dropping "support" for cpu_pm=on setups.
next prev parent reply other threads:[~2026-04-07 18:24 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-04-07 18:11 Bug with nested PAUSE intercept on SVM Kaplan, David
2026-04-07 18:24 ` Sean Christopherson [this message]
2026-04-07 18:30 ` Kaplan, David
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