KVM: VMX: Allow guest to set DEBUGCTL.RTM_DEBUG if RTM is supported

commit 17ec2f965344ee3fd6620bef7ef68792f4ac3af0 upstream.

Let the guest set DEBUGCTL.RTM_DEBUG if RTM is supported according to the
guest CPUID model, as debug support is supposed to be available if RTM is
supported, and there are no known downsides to letting the guest debug RTM
aborts.

Note, there are no known bug reports related to RTM_DEBUG, the primary
motivation is to reduce the probability of breaking existing guests when a
future change adds a missing consistency check on vmcs12.GUEST_DEBUGCTL
(KVM currently lets L2 run with whatever hardware supports; whoops).

Note #2, KVM already emulates DR6.RTM, and doesn't restrict access to
DR7.RTM.

Fixes: 83c529151ab0 ("KVM: x86: expose Intel cpu new features (HLE, RTM) to guest")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250610232010.162191-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 5cfb5d74dd5f58b367497877a462a8a84439780e..c29127ac626ac41ea8e84c3ca203c34c836b9f22 100644 (file)
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -419,6 +419,7 @@
 #define DEBUGCTLMSR_FREEZE_PERFMON_ON_PMI      (1UL << 12)
 #define DEBUGCTLMSR_FREEZE_IN_SMM_BIT  14
 #define DEBUGCTLMSR_FREEZE_IN_SMM      (1UL << DEBUGCTLMSR_FREEZE_IN_SMM_BIT)
+#define DEBUGCTLMSR_RTM_DEBUG          BIT(15)
 
 #define MSR_PEBS_FRONTEND              0x000003f7
 

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 309b0b400a5a228a02eeaadf2ee36972c7fbe707..91fbddbbc3ba7ca5e4c779f4f7155005f79a3ee1 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2186,6 +2186,10 @@ static u64 vmx_get_supported_debugctl(struct kvm_vcpu *vcpu, bool host_initiated
            (host_initiated || intel_pmu_lbr_is_enabled(vcpu)))
                debugctl |= DEBUGCTLMSR_LBR | DEBUGCTLMSR_FREEZE_LBRS_ON_PMI;
 
+       if (boot_cpu_has(X86_FEATURE_RTM) &&
+           (host_initiated || guest_cpu_cap_has(vcpu, X86_FEATURE_RTM)))
+               debugctl |= DEBUGCTLMSR_RTM_DEBUG;
+
        return debugctl;
 }