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.

But actually, I don't quite see how that works reliably for interrupts
injected with KVM_INTERRUPT either. In kvm_vcpu_ioctl_interrupt() the
KVM_REQ_EVENT request is set once, but that'll get cleared the first time
through vcpu_enter_guest(). So if the first exit is for something *else*
without interrupts being enabled yet, won't the KVM_REQ_EVENT request
have been consumed already and just be lost?

I wonder if my addition of '|| kvm_xen_has_interrupt(vcpu)' should
actually be '|| kvm_has_injectable_intr(vcpu)' to fix that pre-existing
bug?

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 62f8dc78807bfa7da709294a84fddc0087606745..36f2a2756bcc342560b77e0e828a0e2cfecc33e7 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -902,6 +902,7 @@ struct msr_bitmap_range {
 /* Xen emulation context */
 struct kvm_xen {
        bool long_mode;
+       u8 upcall_vector;
        struct kvm_host_map shinfo_map;
        void *shinfo;
 };

diff --git a/arch/x86/kvm/irq.c b/arch/x86/kvm/irq.c
index 814698e5b1526180da42043045e87a521870698d..24668b51b5c894c6b34d55c48225a5e784467f86 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 ad9eea8f4f2680c7f89434c93a54fb2c32a149eb..1711072b36165e29ed39057cefb87b6808206b55 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8891,7 +8891,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
                        kvm_x86_ops.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 8a807b610d2bd22aa6f4f58bc10041df4594d5c5..fd7164e641bdf2d595692ac3d60a03e5505708c0 100644 (file)
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -257,6 +257,22 @@ void kvm_xen_setup_pvclock_page(struct kvm_vcpu *v)
        srcu_read_unlock(&v->kvm->srcu, idx);
 }
 
+int kvm_xen_has_interrupt(struct kvm_vcpu *v)
+{
+       int rc = 0;
+
+       if (v->kvm->arch.xen.upcall_vector) {
+               int idx = srcu_read_lock(&v->kvm->srcu);
+               struct vcpu_info *vcpu_info = xen_vcpu_info(v);
+
+               if (vcpu_info && READ_ONCE(vcpu_info->evtchn_upcall_pending))
+                       rc = 1;
+
+               srcu_read_unlock(&v->kvm->srcu, idx);
+       }
+       return rc;
+}
+
 static int vcpu_attr_loc(struct kvm_vcpu *vcpu, u16 type,
                         struct kvm_host_map **map, void ***hva, size_t *sz)
 {
@@ -335,6 +351,14 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
                break;
        }
 
+       case KVM_XEN_ATTR_TYPE_UPCALL_VECTOR:
+               if (data->u.vector < 0x10)
+                       return -EINVAL;
+
+               kvm->arch.xen.upcall_vector = data->u.vector;
+               r = 0;
+               break;
+
        default:
                break;
        }
@@ -382,6 +406,11 @@ 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 ccd6002f55bc029ef2fb4bab6a1425f91293aa7d..1f599342f02cfefabc3d7625b3293f9a522327a9 100644 (file)
--- a/arch/x86/kvm/xen.h
+++ b/arch/x86/kvm/xen.h
@@ -25,6 +25,7 @@ static inline struct kvm_vcpu *xen_vcpu_to_vcpu(struct kvm_vcpu_xen *xen_vcpu)
 void kvm_xen_setup_pvclock_page(struct kvm_vcpu *vcpu);
 void kvm_xen_setup_runstate_page(struct kvm_vcpu *vcpu);
 void kvm_xen_runstate_set_preempted(struct kvm_vcpu *vcpu);
+int kvm_xen_has_interrupt (struct kvm_vcpu *vcpu);
 int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
 int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
 int kvm_xen_hypercall(struct kvm_vcpu *vcpu);

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 1047364d1adffe54d85776deae2fa134e7f1f112..113279fa9e1e3cae822fcb0dce047bbddc0a3254 100644 (file)
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1587,6 +1587,7 @@ struct kvm_xen_hvm_attr {
 
        union {
                __u8 long_mode;
+               __u8 vector;
                struct {
                        __u64 gfn;
                } shared_info;
@@ -1604,6 +1605,7 @@ struct kvm_xen_hvm_attr {
 #define KVM_XEN_ATTR_TYPE_VCPU_INFO            0x2
 #define KVM_XEN_ATTR_TYPE_VCPU_TIME_INFO       0x3
 #define KVM_XEN_ATTR_TYPE_VCPU_RUNSTATE                0x4
+#define KVM_XEN_ATTR_TYPE_UPCALL_VECTOR                0x5
 
 /* Secure Encrypted Virtualization command */
 enum sev_cmd_id {