KVM: SVM: Handle #MCs in guest outside of fastpath

Handle Machine Checks (#MC) that happen in the guest (by forwarding them
to the host) outside of KVM's fastpath so that as much host state as
possible is re-loaded before invoking the kernel's #MC handler.  The only
requirement is that KVM invokes the #MC handler before enabling IRQs (and
even that could _probably_ be relaxed to handling #MCs before enabling
preemption).

Waiting to handle #MCs until "more" host state is loaded hardens KVM
against flaws in the #MC handler, which has historically been quite
brittle. E.g. prior to commit 5567d11 ("x86/mce: Send #MC singal from
task work"), the #MC code could trigger a schedule() with IRQs and
preemption disabled.  That led to a KVM hack-a-fix in commit 1811d97
("x86/kvm: move kvm_load/put_guest_xcr0 into atomic context").

Note, except for #MCs on VM-Enter, VMX already handles #MCs outside of the
fastpath.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-By: Jon Kohler <jon@nutanix.com>
Link: https://patch.msgid.link/20251030224246.3456492-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/svm.c

@@ -4325,14 +4325,6 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
 
 	vcpu->arch.regs_avail &= ~SVM_REGS_LAZY_LOAD_SET;
 
-	/*
-	 * We need to handle MC intercepts here before the vcpu has a chance to
-	 * change the physical cpu
-	 */
-	if (unlikely(svm->vmcb->control.exit_code ==
-		     SVM_EXIT_EXCP_BASE + MC_VECTOR))
-		svm_handle_mce(vcpu);
-
 	trace_kvm_exit(vcpu, KVM_ISA_SVM);
 
 	svm_complete_interrupts(vcpu);
@@ -4621,8 +4613,16 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
 
 static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
 {
-	if (to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_INTR)
+	switch (to_svm(vcpu)->vmcb->control.exit_code) {
+	case SVM_EXIT_EXCP_BASE + MC_VECTOR:
+		svm_handle_mce(vcpu);
+		break;
+	case SVM_EXIT_INTR:
 		vcpu->arch.at_instruction_boundary = true;
+		break;
+	default:
+		break;
+	}
 }
 
 static void svm_setup_mce(struct kvm_vcpu *vcpu)