kvm: x86: Introduce KVM_CAP_EXCEPTION_PAYLOAD

This is a per-VM capability which can be enabled by userspace so that
the faulting linear address will be included with the information
about a pending #PF in L2, and the "new DR6 bits" will be included
with the information about a pending #DB in L2. With this capability
enabled, the L1 hypervisor can now intercept #PF before CR2 is
modified. Under VMX, the L1 hypervisor can now intercept #DB before
DR6 and DR7 are modified.

When userspace has enabled KVM_CAP_EXCEPTION_PAYLOAD, it should
generally provide an appropriate payload when injecting a #PF or #DB
exception via KVM_SET_VCPU_EVENTS. However, to support restoring old
checkpoints, this payload is not required.

Note that bit 16 of the "new DR6 bits" is set to indicate that a debug
exception (#DB) or a breakpoint exception (#BP) occurred inside an RTM
region while advanced debugging of RTM transactional regions was
enabled. This is the reverse of DR6.RTM, which is cleared in this
scenario.

This capability also enables exception.pending in struct
kvm_vcpu_events, which allows userspace to distinguish between pending
and injected exceptions.
Reported-by: NJim Mattson <jmattson@google.com>
Suggested-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NJim Mattson <jmattson@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -4568,7 +4568,7 @@ hpage module parameter is not set to 1, -EINVAL is returned.
 While it is generally possible to create a huge page backed VM without
 this capability, the VM will not be able to run.
 
-7.14 KVM_CAP_MSR_PLATFORM_INFO
+7.15 KVM_CAP_MSR_PLATFORM_INFO
 
 Architectures: x86
 Parameters: args[0] whether feature should be enabled or not
@@ -4591,6 +4591,31 @@ state).  Enabling this capability on a VM depends on the CPU having
 the necessary functionality and on the facility being enabled with a
 kvm-hv module parameter.
 
+7.17 KVM_CAP_EXCEPTION_PAYLOAD
+
+Architectures: x86
+Parameters: args[0] whether feature should be enabled or not
+
+With this capability enabled, CR2 will not be modified prior to the
+emulated VM-exit when L1 intercepts a #PF exception that occurs in
+L2. Similarly, for kvm-intel only, DR6 will not be modified prior to
+the emulated VM-exit when L1 intercepts a #DB exception that occurs in
+L2. As a result, when KVM_GET_VCPU_EVENTS reports a pending #PF (or
+#DB) exception for L2, exception.has_payload will be set and the
+faulting address (or the new DR6 bits*) will be reported in the
+exception_payload field. Similarly, when userspace injects a #PF (or
+#DB) into L2 using KVM_SET_VCPU_EVENTS, it is expected to set
+exception.has_payload and to put the faulting address (or the new DR6
+bits*) in the exception_payload field.
+
+This capability also enables exception.pending in struct
+kvm_vcpu_events, which allows userspace to distinguish between pending
+and injected exceptions.
+
+
+* For the new DR6 bits, note that bit 16 is set iff the #DB exception
+  will clear DR6.RTM.
+
 8. Other capabilities.
 ----------------------
 

arch/x86/kvm/x86.c

@@ -3015,6 +3015,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_IMMEDIATE_EXIT:
 	case KVM_CAP_GET_MSR_FEATURES:
 	case KVM_CAP_MSR_PLATFORM_INFO:
+	case KVM_CAP_EXCEPTION_PAYLOAD:
 		r = 1;
 		break;
 	case KVM_CAP_SYNC_REGS:
@@ -4500,6 +4501,10 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		kvm->arch.guest_can_read_msr_platform_info = cap->args[0];
 		r = 0;
 		break;
+	case KVM_CAP_EXCEPTION_PAYLOAD:
+		kvm->arch.exception_payload_enabled = cap->args[0];
+		r = 0;
+		break;
 	default:
 		r = -EINVAL;
 		break;

include/uapi/linux/kvm.h

@@ -964,6 +964,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_HYPERV_SEND_IPI 161
 #define KVM_CAP_COALESCED_PIO 162
 #define KVM_CAP_HYPERV_ENLIGHTENED_VMCS 163
+#define KVM_CAP_EXCEPTION_PAYLOAD 164
 
 #ifdef KVM_CAP_IRQ_ROUTING