KVM: SVM: Limit incorrect check on SVM_EXIT_ERR to running as a VM

Limit KVM's incorrect check for VMXEXIT_INVALID, a.k.a. SVM_EXIT_ERR, to
running as a VM, as detected by X86_FEATURE_HYPERVISOR.  The exit_code and
all failure codes, e.g. VMXEXIT_INVALID, are 64-bit values, and so checking
only bits 31:0 could result in false positives when running on non-broken
hardware, e.g. in the extremely unlikely scenario exit code 0xffffffffull
is ever generated by hardware.

Keep the 32-bit check to play nice with running on broken KVM (for years,
KVM has not set bits 63:32 when synthesizing nested SVM VM-Exits).

Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251230211347.4099600-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/svm.h

@@ -426,7 +426,10 @@ static __always_inline struct vcpu_svm *to_svm(struct kvm_vcpu *vcpu)
 
 static inline bool svm_is_vmrun_failure(u64 exit_code)
 {
-	return (u32)exit_code == (u32)SVM_EXIT_ERR;
+	if (cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
+		return (u32)exit_code == (u32)SVM_EXIT_ERR;
+
+	return exit_code == SVM_EXIT_ERR;
 }
 
 /*