linux-kernel - [PATCH 0/3] KVM: SVM: Zero DEBUGCTL before VMRUN if necessary

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Message-ID: <20250224181315.2376869-1-seanjc@google.com>
Date: Mon, 24 Feb 2025 10:13:12 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, rangemachine@...il.com, 
	whanos@...gal.fun, Ravi Bangoria <ravi.bangoria@....com>, 
	Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH 0/3] KVM: SVM: Zero DEBUGCTL before VMRUN if necessary

PeterZ,

Can you confirm that the last patch (snapshot and restore DEBUGCTL with
IRQs disabled) is actually necessary?  I'm 99% certain it is, but I'm
holding out hope that it somehow isn't, because I don't love the idea of
adding a RDMSR to every VM-Entry.

Assuming DEBUGCTL can indeed get modified in IRQ context, it probably
makes sense to add a per-CPU cache to eliminate the RDMSR.  Unfortunately,
there are quite a few open-coded WRMSRs, so it's not a trivial change.

On to the main event...

Fix a long-lurking bug in SVM where KVM runs the guest with the host's
DEBUGCTL if LBR virtualization is disabled.  AMD CPUs rather stupidly
context switch DEBUGCTL if and only if LBR virtualization is enabled (not
just supported, but fully enabled).

The bug has gone unnoticed because until recently, the only bits that
KVM would leave set were things like BTF, which are guest visible but
won't cause functional problems unless guest software is being especially
particular about #DBs.

The bug was exposed by the addition of BusLockTrap ("Detect" in the kernel),
as the resulting #DBs due to split-lock accesses in guest userspace (lol
Steam) get reflected into the guest by KVM.

Sean Christopherson (3):
  KVM: x86: Snapshot the host's DEBUGCTL in common x86
  KVM: SVM: Manually zero/restore DEBUGCTL if LBR virtualization is
    disabled
  KVM: x86: Snapshot the host's DEBUGCTL after disabling IRQs

 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/svm/svm.c          | 14 ++++++++++++++
 arch/x86/kvm/vmx/vmx.c          |  8 ++------
 arch/x86/kvm/vmx/vmx.h          |  2 --
 arch/x86/kvm/x86.c              |  2 ++
 5 files changed, 19 insertions(+), 8 deletions(-)

base-commit: fed48e2967f402f561d80075a20c5c9e16866e53
-- 
2.48.1.658.g4767266eb4-goog