[PATCH v5 33/34] KVM: x86/vmx: call external_interrupt() to handle IRQ in IRQ caused VM exits

[Xin Li <xin3.li@xxxxxxxxx>]

When FRED is enabled, IDT is gone, thus call external_interrupt() to handle
IRQ in IRQ caused VM exits.

Create an event return stack frame with the host context immediately after
a VM exit for calling external_interrupt(). All other fields of the pt_regs
structure are cleared to 0. Refer to the discussion about the register values
in the pt_regs structure at:

  https://lore.kernel.org/kvm/ef2c54f7-14b9-dcbb-c3c4-1533455e7a18@xxxxxxxxxx/

Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
---

Changes since v4:
*) Do NOT use the term "injection", which in the KVM context means to
   reinject an event into the guest (Sean Christopherson).
*) Use cs/ss instead of csx/ssx when initializing the pt_regs structure
   for calling external_interrupt(), otherwise it breaks i386 build.
---
 arch/x86/kvm/vmx/vmx.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index bcac3efcde41..3ebeaab34b2e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -47,6 +47,7 @@
 #include <asm/mshyperv.h>
 #include <asm/mwait.h>
 #include <asm/spec-ctrl.h>
+#include <asm/traps.h>
 #include <asm/virtext.h>
 #include <asm/vmx.h>
 
@@ -6923,7 +6924,26 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
 		return;
 
 	kvm_before_interrupt(vcpu, KVM_HANDLING_IRQ);
-	vmx_do_interrupt_irqoff(gate_offset(desc));
+	if (cpu_feature_enabled(X86_FEATURE_FRED)) {
+		struct vcpu_vmx *vmx = to_vmx(vcpu);
+		struct pt_regs regs = {};
+
+		/*
+		 * Create an event return stack frame with the
+		 * host context immediately after a VM exit.
+		 *
+		 * All other fields of the pt_regs structure are
+		 * cleared to 0.
+		 */
+		regs.ss		= __KERNEL_DS;
+		regs.sp		= vmx->loaded_vmcs->host_state.rsp;
+		regs.flags	= X86_EFLAGS_FIXED;
+		regs.cs		= __KERNEL_CS;
+		regs.ip		= (unsigned long)vmx_vmexit;
+
+		external_interrupt(&regs, vector);
+	} else
+		vmx_do_interrupt_irqoff(gate_offset(desc));
 	kvm_after_interrupt(vcpu);
 
 	vcpu->arch.at_instruction_boundary = true;
-- 
2.34.1