KVM: x86/xen: Add event channel interrupt vector upcall

It turns out that we can't handle event channels *entirely* in userspace
by delivering them as ExtINT, because KVM is a bit picky about when it
accepts ExtINT interrupts from a legacy PIC. The in-kernel local APIC
has to have LVT0 configured in APIC_MODE_EXTINT and unmasked, which
isn't necessarily the case for Xen guests especially on secondary CPUs.

To cope with this, add kvm_xen_get_interrupt() which checks the
evtchn_pending_upcall field in the Xen vcpu_info, and delivers the Xen
upcall vector (configured by KVM_XEN_ATTR_TYPE_UPCALL_VECTOR) if it's
set regardless of LAPIC LVT0 configuration. This gives us the minimum
support we need for completely userspace-based implementation of event
channels.

This does mean that vcpu_enter_guest() needs to check for the
evtchn_pending_upcall flag being set, because it can't rely on someone
having set KVM_REQ_EVENT unless we were to add some way for userspace to
do so manually.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c6c84c4..b37afd8 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -913,6 +913,7 @@ struct msr_bitmap_range {
 struct kvm_xen {
        bool long_mode;
        bool shinfo_set;
+       u8 upcall_vector;
        struct gfn_to_hva_cache shinfo_cache;
 };
 

diff --git a/arch/x86/kvm/irq.c b/arch/x86/kvm/irq.c
index a035cca..172b053 100644 (file)
--- a/arch/x86/kvm/irq.c
+++ b/arch/x86/kvm/irq.c
@@ -14,6 +14,7 @@
 #include "irq.h"
 #include "i8254.h"
 #include "x86.h"
+#include "xen.h"
 
 /*
  * check if there are pending timer events
@@ -56,6 +57,9 @@ int kvm_cpu_has_extint(struct kvm_vcpu *v)
        if (!lapic_in_kernel(v))
                return v->arch.interrupt.injected;
 
+       if (kvm_xen_has_interrupt(v))
+               return 1;
+
        if (!kvm_apic_accept_pic_intr(v))
                return 0;
 
@@ -110,6 +114,9 @@ static int kvm_cpu_get_extint(struct kvm_vcpu *v)
        if (!lapic_in_kernel(v))
                return v->arch.interrupt.nr;
 
+       if (kvm_xen_has_interrupt(v))
+               return v->kvm->arch.xen.upcall_vector;
+
        if (irqchip_split(v->kvm)) {
                int vector = v->arch.pending_external_vector;
 

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0f64843..1cedbc3 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8987,7 +8987,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
                        static_call(kvm_x86_msr_filter_changed)(vcpu);
        }
 
-       if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
+       if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win ||
+           kvm_xen_has_interrupt(vcpu)) {
                ++vcpu->stat.req_event;
                kvm_apic_accept_events(vcpu);
                if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) {

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index bd34322..39a7ffc 100644 (file)
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -61,6 +61,44 @@ out:
        return ret;
 }
 
+int __kvm_xen_has_interrupt(struct kvm_vcpu *v)
+{
+       u8 rc = 0;
+
+       /*
+        * If the global upcall vector (HVMIRQ_callback_vector) is set and
+        * the vCPU's evtchn_upcall_pending flag is set, the IRQ is pending.
+        */
+       struct gfn_to_hva_cache *ghc = &v->arch.xen.vcpu_info_cache;
+       struct kvm_memslots *slots = kvm_memslots(v->kvm);
+       unsigned int offset = offsetof(struct vcpu_info, evtchn_upcall_pending);
+
+       /* No need for compat handling here */
+       BUILD_BUG_ON(offsetof(struct vcpu_info, evtchn_upcall_pending) !=
+                    offsetof(struct compat_vcpu_info, evtchn_upcall_pending));
+       BUILD_BUG_ON(sizeof(rc) !=
+                    sizeof(((struct vcpu_info *)0)->evtchn_upcall_pending));
+       BUILD_BUG_ON(sizeof(rc) !=
+                    sizeof(((struct compat_vcpu_info *)0)->evtchn_upcall_pending));
+
+       /*
+        * For efficiency, this mirrors the checks for using the valid
+        * cache in kvm_read_guest_offset_cached(), but just uses
+        * __get_user() instead. And falls back to the slow path.
+        */
+       if (likely(slots->generation == ghc->generation &&
+                  !kvm_is_error_hva(ghc->hva) && ghc->memslot)) {
+               /* Fast path */
+               __get_user(rc, (u8 __user *)ghc->hva + offset);
+       } else {
+               /* Slow path */
+               kvm_read_guest_offset_cached(v->kvm, ghc, &rc, offset,
+                                            sizeof(rc));
+       }
+
+       return rc;
+}
+
 int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 {
        int r = -ENOENT;
@@ -83,6 +121,16 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
                r = kvm_xen_shared_info_init(kvm, data->u.shared_info.gfn);
                break;
 
+
+       case KVM_XEN_ATTR_TYPE_UPCALL_VECTOR:
+               if (data->u.vector < 0x10)
+                       r = -EINVAL;
+               else {
+                       kvm->arch.xen.upcall_vector = data->u.vector;
+                       r = 0;
+               }
+               break;
+
        default:
                break;
        }
@@ -110,6 +158,11 @@ int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
                }
                break;
 
+       case KVM_XEN_ATTR_TYPE_UPCALL_VECTOR:
+               data->u.vector = kvm->arch.xen.upcall_vector;
+               r = 0;
+               break;
+
        default:
                break;
        }

diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
index fb85377..4b32489 100644 (file)
--- a/arch/x86/kvm/xen.h
+++ b/arch/x86/kvm/xen.h
@@ -13,6 +13,7 @@
 
 extern struct static_key_false_deferred kvm_xen_enabled;
 
+int __kvm_xen_has_interrupt(struct kvm_vcpu *vcpu);
 int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data);
 int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data);
 int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
@@ -29,6 +30,14 @@ static inline bool kvm_xen_hypercall_enabled(struct kvm *kvm)
                 KVM_XEN_HVM_CONFIG_INTERCEPT_HCALL);
 }
 
+static inline int kvm_xen_has_interrupt(struct kvm_vcpu *vcpu)
+{
+       if (static_branch_unlikely(&kvm_xen_enabled.key) &&
+           vcpu->arch.xen.vcpu_info_set && vcpu->kvm->arch.xen.upcall_vector)
+               return __kvm_xen_has_interrupt(vcpu);
+
+       return 0;
+}
 
 /* 32-bit compatibility definitions, also used natively in 32-bit build */
 #include <asm/pvclock-abi.h>

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 0f045ff..4d4cd00 100644 (file)
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1596,6 +1596,7 @@ struct kvm_xen_hvm_attr {
        __u16 pad[3];
        union {
                __u8 long_mode;
+               __u8 vector;
                struct {
                        __u64 gfn;
                } shared_info;
@@ -1605,6 +1606,7 @@ struct kvm_xen_hvm_attr {
 
 #define KVM_XEN_ATTR_TYPE_LONG_MODE            0x0
 #define KVM_XEN_ATTR_TYPE_SHARED_INFO          0x1
+#define KVM_XEN_ATTR_TYPE_UPCALL_VECTOR                0x2
 
 /* Per-vCPU Xen attributes */
 #define KVM_XEN_VCPU_GET_ATTR  _IOWR(KVMIO, 0xca, struct kvm_xen_vcpu_attr)