[PATCH v2 13/23] kvm: x86: Intercept #NM for saving IA32_XFD_ERR

[Jing Liu <jing2.liu@intel.com>]

Guest IA32_XFD_ERR is generally modified in two places:

  - Set by CPU when #NM is triggered;
  - Cleared by guest in its #NM handler;

Intercept #NM for the first case, if guest CPUID includes any dynamic
xfeature. #NM is rare if the guest doesn't use dynamic features.
Otherwise, there is at most one exception per guest task given a
dynamic feature.

Save the current XFD_ERR value to the guest_fpu container in the #NM
VM-exit handler. This must be done with interrupt/preemption disabled,
otherwise the unsaved MSR value may be clobbered by host operations.

Inject a virtual #NM to the guest after saving the MSR value.

Restore the host value (always ZERO outside of the host #NM
handler) before enabling preemption.

Restore the guest value from the guest_fpu container right before
entering the guest (with preemption disabled).

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jing Liu <jing2.liu@intel.com>
---
TODO: Investigate delaying #NM interception until guest sets a dynamic
feature in XCR0.

 arch/x86/kvm/vmx/vmcs.h |  5 +++++
 arch/x86/kvm/vmx/vmx.c  | 15 ++++++++++++++-
 arch/x86/kvm/x86.c      |  6 ++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmcs.h b/arch/x86/kvm/vmx/vmcs.h
index 6e5de2e2b0da..c57798b56f95 100644
--- a/arch/x86/kvm/vmx/vmcs.h
+++ b/arch/x86/kvm/vmx/vmcs.h
@@ -129,6 +129,11 @@ static inline bool is_machine_check(u32 intr_info)
 	return is_exception_n(intr_info, MC_VECTOR);
 }
 
+static inline bool is_nm(u32 intr_info)
+{
+	return is_exception_n(intr_info, NM_VECTOR);
+}
+
 /* Undocumented: icebp/int1 */
 static inline bool is_icebp(u32 intr_info)
 {
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 9453743ce0c4..483075045253 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -36,6 +36,7 @@
 #include <asm/debugreg.h>
 #include <asm/desc.h>
 #include <asm/fpu/api.h>
+#include <asm/fpu/xstate.h>
 #include <asm/idtentry.h>
 #include <asm/io.h>
 #include <asm/irq_remapping.h>
@@ -763,6 +764,9 @@ void vmx_update_exception_bitmap(struct kvm_vcpu *vcpu)
 		vmcs_write32(PAGE_FAULT_ERROR_CODE_MATCH, match);
 	}
 
+	if (vcpu->arch.guest_supported_xcr0 & XFEATURE_MASK_USER_DYNAMIC)
+		eb |= (1u << NM_VECTOR);
+
 	vmcs_write32(EXCEPTION_BITMAP, eb);
 }
 
@@ -4750,7 +4754,7 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 	vect_info = vmx->idt_vectoring_info;
 	intr_info = vmx_get_intr_info(vcpu);
 
-	if (is_machine_check(intr_info) || is_nmi(intr_info))
+	if (is_machine_check(intr_info) || is_nmi(intr_info) || is_nm(intr_info))
 		return 1; /* handled by handle_exception_nmi_irqoff() */
 
 	if (is_invalid_opcode(intr_info))
@@ -6338,6 +6342,12 @@ static void handle_interrupt_nmi_irqoff(struct kvm_vcpu *vcpu,
 	kvm_after_interrupt(vcpu);
 }
 
+static void handle_exception_nm(struct kvm_vcpu *vcpu)
+{
+	rdmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
+	kvm_queue_exception(vcpu, NM_VECTOR);
+}
+
 static void handle_exception_nmi_irqoff(struct vcpu_vmx *vmx)
 {
 	const unsigned long nmi_entry = (unsigned long)asm_exc_nmi_noist;
@@ -6346,6 +6356,9 @@ static void handle_exception_nmi_irqoff(struct vcpu_vmx *vmx)
 	/* if exit due to PF check for async PF */
 	if (is_page_fault(intr_info))
 		vmx->vcpu.arch.apf.host_apf_flags = kvm_read_and_reset_apf_flags();
+	/* if exit due to NM, handle before preemptions are enabled */
+	else if (is_nm(intr_info))
+		handle_exception_nm(&vmx->vcpu);
 	/* Handle machine checks before interrupts are enabled */
 	else if (is_machine_check(intr_info))
 		kvm_machine_check();
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a274146ef439..e528085030b3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9894,6 +9894,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
 		switch_fpu_return();
 
+	if (vcpu->arch.guest_fpu.xfd_err)
+		wrmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
+
 	if (unlikely(vcpu->arch.switch_db_regs)) {
 		set_debugreg(0, 7);
 		set_debugreg(vcpu->arch.eff_db[0], 0);
@@ -9957,6 +9960,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 
 	static_call(kvm_x86_handle_exit_irqoff)(vcpu);
 
+	if (vcpu->arch.guest_fpu.xfd_err)
+		wrmsrl(MSR_IA32_XFD_ERR, 0);
+
 	/*
 	 * Consume any pending interrupts, including the possible source of
 	 * VM-Exit on SVM and any ticks that occur between VM-Exit and now.
-- 
2.27.0