kvm: x86: Intercept #NM for saving IA32_XFD_ERR

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 when a nonzero value is written
to IA32_XFD. Nonzero indicates that the guest is willing to do
dynamic fpstate expansion for certain xfeatures, thus KVM needs to
manage and virtualize guest XFD_ERR properly. The vcpu exception
bitmap is updated in XFD write emulation according to guest_fpu::xfd.

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

The saving operation is conducted conditionally only when guest_fpu:xfd
includes a non-zero value. Doing so also avoids misread on a platform
which doesn't support XFD but #NM is triggered due to L1 interception.

Queueing #NM to the guest is postponed to handle_exception_nmi(). This
goes through the nested_vmx check so a virtual vmexit is queued instead
when #NM is triggered in L2 but L1 wants to intercept it.

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

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

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jing Liu <jing2.liu@intel.com>
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20220105123532.12586-13-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx/vmcs.h b/arch/x86/kvm/vmx/vmcs.h
index 6e5de2e2b0da6d997ae720e96ab69ab2bfbcc001..e325c290a8162c18c5e083d7f5dc547d7f586a79 100644 (file)
--- 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_fault(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 7b5abe25e1e5dcac094399c467c0a9999480f6df..84f6904cdb6e2d6ad7272d4132702167b3350c4f 100644 (file)
--- 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>
@@ -761,6 +762,13 @@ void vmx_update_exception_bitmap(struct kvm_vcpu *vcpu)
                vmcs_write32(PAGE_FAULT_ERROR_CODE_MATCH, match);
        }
 
+       /*
+        * Trap #NM if guest xfd contains a non-zero value so guest XFD_ERR
+        * can be saved timely.
+        */
+       if (vcpu->arch.guest_fpu.fpstate->xfd)
+               eb |= (1u << NM_VECTOR);
+
        vmcs_write32(EXCEPTION_BITMAP, eb);
 }
 
@@ -1967,6 +1975,12 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
        case MSR_KERNEL_GS_BASE:
                vmx_write_guest_kernel_gs_base(vmx, data);
                break;
+       case MSR_IA32_XFD:
+               ret = kvm_set_msr_common(vcpu, msr_info);
+               /* Update #NM interception according to guest xfd */
+               if (!ret)
+                       vmx_update_exception_bitmap(vcpu);
+               break;
 #endif
        case MSR_IA32_SYSENTER_CS:
                if (is_guest_mode(vcpu))
@@ -4798,6 +4812,17 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
        if (is_machine_check(intr_info) || is_nmi(intr_info))
                return 1; /* handled by handle_exception_nmi_irqoff() */
 
+       /*
+        * Queue the exception here instead of in handle_nm_fault_irqoff().
+        * This ensures the nested_vmx check is not skipped so vmexit can
+        * be reflected to L1 (when it intercepts #NM) before reaching this
+        * point.
+        */
+       if (is_nm_fault(intr_info)) {
+               kvm_queue_exception(vcpu, NM_VECTOR);
+               return 1;
+       }
+
        if (is_invalid_opcode(intr_info))
                return handle_ud(vcpu);
 
@@ -6399,6 +6424,26 @@ static void handle_interrupt_nmi_irqoff(struct kvm_vcpu *vcpu,
        kvm_after_interrupt(vcpu);
 }
 
+static void handle_nm_fault_irqoff(struct kvm_vcpu *vcpu)
+{
+       /*
+        * Save xfd_err to guest_fpu before interrupt is enabled, so the
+        * MSR value is not clobbered by the host activity before the guest
+        * has chance to consume it.
+        *
+        * Do not blindly read xfd_err here, since this exception might
+        * be caused by L1 interception on a platform which doesn't
+        * support xfd at all.
+        *
+        * Do it conditionally upon guest_fpu::xfd. xfd_err matters
+        * only when xfd contains a non-zero value.
+        *
+        * Queuing exception is done in vmx_handle_exit. See comment there.
+        */
+       if (vcpu->arch.guest_fpu.fpstate->xfd)
+               rdmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
+}
+
 static void handle_exception_nmi_irqoff(struct vcpu_vmx *vmx)
 {
        const unsigned long nmi_entry = (unsigned long)asm_exc_nmi_noist;
@@ -6407,6 +6452,9 @@ 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 interrupts are enabled */
+       else if (is_nm_fault(intr_info))
+               handle_nm_fault_irqoff(&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 b18d2838606f8717c20b58aba65209b3226103f2..bb9534590a3a9a267ec203b98b2daff4a1104d16 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9952,6 +9952,9 @@ 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);
@@ -10015,6 +10018,9 @@ 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.