KVM: SVM: Allow guest writes to set MSR_AMD64_DE_CFG bits

JIRA: https://issues.redhat.com/browse/RHEL-71416

commit 4d5163c
Author: Sean Christopherson <seanjc@google.com>
Date:   Wed Dec 11 09:29:52 2024 -0800

    KVM: SVM: Allow guest writes to set MSR_AMD64_DE_CFG bits

    Drop KVM's arbitrary behavior of making DE_CFG.LFENCE_SERIALIZE read-only
    for the guest, as rejecting writes can lead to guest crashes, e.g. Windows
    in particular doesn't gracefully handle unexpected #GPs on the WRMSR, and
    nothing in the AMD manuals suggests that LFENCE_SERIALIZE is read-only _if
    it exists_.

    KVM only allows LFENCE_SERIALIZE to be set, by the guest or host, if the
    underlying CPU has X86_FEATURE_LFENCE_RDTSC, i.e. if LFENCE is guaranteed
    to be serializing.  So if the guest sets LFENCE_SERIALIZE, KVM will provide
    the desired/correct behavior without any additional action (the guest's
    value is never stuffed into hardware).  And having LFENCE be serializing
    even when it's not _required_ to be is a-ok from a functional perspective.

    Fixes: 74a0e79 ("KVM: SVM: Disallow guest from changing userspace's MSR_AMD64_DE_CFG value")
    Fixes: d1d93fa ("KVM: SVM: Add MSR-based feature support for serializing LFENCE")
    Reported-by: Simon Pilkington <simonp.git@mailbox.org>
    Closes: https://lore.kernel.org/all/52914da7-a97b-45ad-86a0-affdf8266c61@mailbox.org
    Cc: Tom Lendacky <thomas.lendacky@amd.com>
    Cc: stable@vger.kernel.org
    Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
    Link: https://lore.kernel.org/r/20241211172952.1477605-1-seanjc@google.com
    Signed-off-by: Sean Christopherson <seanjc@google.com>

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Author: vittyvk

arch/x86/kvm/svm/svm.c

@@ -3199,15 +3199,6 @@ 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;
 	}