linux-kernel - [PATCH] KVM: SVM: allow flipping the LFENCE

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Message-ID: <20241219103837.325113-1-pbonzini@redhat.com>
Date: Thu, 19 Dec 2024 05:38:37 -0500
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
	stable@...r.kernel.org,
	Tom Lendacky <thomas.lendacky@....com>
Subject: [PATCH] KVM: SVM: allow flipping the LFENCE_SERIALIZE bit

Allow the guest to both clear and set the LFENCE_SERIALIZE bit as long as
it is set in the host.  It is absolutely okay for the guest to set it if
LFENCE_RDTSC is supported but userspace left it cleared; and it is also
acceptable that the guest clears the bit even if this will actually have
no effect.

This fixes booting Windows in some configuration where it tries to set
the bit, and hangs if it does not succeed.

Suggested-by: Sean Christopherson <seanjc@...gle.com>
Fixes: 74a0e79df68a ("KVM: SVM: Disallow guest from changing userspace's MSR_AMD64_DE_CFG value")
Cc: stable@...r.kernel.org
Cc: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm/svm.c | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index dd15cc635655..21dacd312779 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3201,15 +3201,6 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		if (data & ~supported_de_cfg)
 			return 1;
 
-		/*
-		 * Don't let the guest change the host-programmed value.  The
-		 * MSR is very model specific, i.e. contains multiple bits that
-		 * are completely unknown to KVM, and the one bit known to KVM
-		 * is simply a reflection of hardware capabilities.
-		 */
-		if (!msr->host_initiated && data != svm->msr_decfg)
-			return 1;
-
 		svm->msr_decfg = data;
 		break;
 	}
-- 
2.43.5