KVM: VMX: Handle #MCs on VM-Enter/TD-Enter outside of the fastpath

Handle Machine Checks (#MC) that happen on VM-Enter (VMX or TDX) 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 related 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 5567d11c21a1 ("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 1811d979c716
("x86/kvm: move kvm_load/put_guest_xcr0 into atomic context").

Note, vmx_handle_exit_irqoff() is common to VMX and TDX guests.

Cc: Tony Lindgren <tony.lindgren@linux.intel.com>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Jon Kohler <jon@nutanix.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Link: https://patch.msgid.link/20251118222328.2265758-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 163f854a39f25e35ca051d330cff83fec845afc3..6d41d2fc8043c9af78dd7167a3833bc60aad6f2c 100644 (file)
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1063,9 +1063,6 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
        if (unlikely((tdx->vp_enter_ret & TDX_SW_ERROR) == TDX_SW_ERROR))
                return EXIT_FASTPATH_NONE;
 
-       if (unlikely(vmx_get_exit_reason(vcpu).basic == EXIT_REASON_MCE_DURING_VMENTRY))
-               kvm_machine_check();
-
        trace_kvm_exit(vcpu, KVM_ISA_VMX);
 
        if (unlikely(tdx_failed_vmentry(vcpu)))

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 634f591d253e7f917ac23c2cab98d046770bc369..788c9e81f2ba5ccd36dd18797630170a48868d3b 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7074,10 +7074,19 @@ void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
        if (to_vt(vcpu)->emulation_required)
                return;
 
-       if (vmx_get_exit_reason(vcpu).basic == EXIT_REASON_EXTERNAL_INTERRUPT)
+       switch (vmx_get_exit_reason(vcpu).basic) {
+       case EXIT_REASON_EXTERNAL_INTERRUPT:
                handle_external_interrupt_irqoff(vcpu, vmx_get_intr_info(vcpu));
-       else if (vmx_get_exit_reason(vcpu).basic == EXIT_REASON_EXCEPTION_NMI)
+               break;
+       case EXIT_REASON_EXCEPTION_NMI:
                handle_exception_irqoff(vcpu, vmx_get_intr_info(vcpu));
+               break;
+       case EXIT_REASON_MCE_DURING_VMENTRY:
+               kvm_machine_check();
+               break;
+       default:
+               break;
+       }
 }
 
 /*
@@ -7526,9 +7535,6 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
        if (unlikely(vmx->fail))
                return EXIT_FASTPATH_NONE;
 
-       if (unlikely((u16)vmx_get_exit_reason(vcpu).basic == EXIT_REASON_MCE_DURING_VMENTRY))
-               kvm_machine_check();
-
        trace_kvm_exit(vcpu, KVM_ISA_VMX);
 
        if (unlikely(vmx_get_exit_reason(vcpu).failed_vmentry))