KVM: SVM: Treat DEBUGCTL[5:2] as reserved

JIRA: https://issues.redhat.com/browse/RHEL-47242

commit 5ecdb48
Author: Sean Christopherson <seanjc@google.com>
Date:   Thu Feb 27 14:24:11 2025 -0800

    KVM: SVM: Treat DEBUGCTL[5:2] as reserved

    Stop ignoring DEBUGCTL[5:2] on AMD CPUs and instead treat them as reserved.
    KVM has never properly virtualized AMD's legacy PBi bits, but did allow
    the guest (and host userspace) to set the bits.  To avoid breaking guests
    when running on CPUs with BusLockTrap, which redefined bit 2 to BLCKDB and
    made bits 5:3 reserved, a previous KVM change ignored bits 5:3, e.g. so
    that legacy guest software wouldn't inadvertently enable BusLockTrap or
    hit a VMRUN failure due to setting reserved.

    To allow for virtualizing BusLockTrap and whatever future features may use
    bits 5:3, treat bits 5:2 as reserved (and hope that doing so doesn't break
    any existing guests).

    Reviewed-and-tested-by: Ravi Bangoria <ravi.bangoria@amd.com>
    Link: https://lore.kernel.org/r/20250227222411.3490595-7-seanjc@google.com
    Signed-off-by: Sean Christopherson <seanjc@google.com>

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Author: Maxim Levitsky

arch/x86/kvm/svm/svm.c

@@ -3233,17 +3233,6 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 			break;
 		}
 
-		/*
-		 * AMD changed the architectural behavior of bits 5:2.  On CPUs
-		 * without BusLockTrap, bits 5:2 control "external pins", but
-		 * on CPUs that support BusLockDetect, bit 2 enables BusLockTrap
-		 * and bits 5:3 are reserved-to-zero.  Sadly, old KVM allowed
-		 * the guest to set bits 5:2 despite not actually virtualizing
-		 * Performance-Monitoring/Breakpoint external pins.  Drop bits
-		 * 5:2 for backwards compatibility.
-		 */
-		data &= ~GENMASK(5, 2);
-
 		/*
 		 * Suppress BTF as KVM doesn't virtualize BTF, but there's no
 		 * way to communicate lack of support to the guest.