[PATCH v1.1 4/5] KVM: Kick resamplefd for split kernel irqchip

[Peter Xu]

This is majorly only for X86 because that's the only one that supports
split irqchip for now.

When the irqchip is split, we face a dilemma that KVM irqfd will be
enabled, however the slow irqchip is still running in the userspace.
It means that the resamplefd in the kernel irqfds won't take any
effect and it can miss to ack INTx interrupts on EOIs.

One example is split irqchip with VFIO INTx, which will break if we
use the VFIO INTx fast path.

This patch can potentially supports the VFIO fast path again for INTx,
that the IRQ delivery will still use the fast path, while we don't
need to trap MMIOs in QEMU for the device to emulate the EIOs (see the
callers of vfio_eoi() hook).  However the EOI of the INTx will still
need to be done from the userspace by caching all the resamplefds in
QEMU and kick properly for IOAPIC EOI broadcast.

When the userspace is responsible for the resamplefd kickup, don't
register it on the kvm_irqfd anymore, because on newer kernels (after
commit 654f1f13ea56, 5.2+) the KVM_IRQFD will fail if with both split
irqchip and resamplefd.  This will make sure that the fast path will
work for all supported kernels.

https://patchwork.kernel.org/patch/10738541/#22609933

Suggested-by: Paolo Bonzini <address@hidden>
Signed-off-by: Peter Xu <address@hidden>
---
v1.1 changelog:
- when resamplefd is going to be kicked from userspace, don't register
  it again in KVM_IRQFD.  Tested against upstream kernel.

 accel/kvm/kvm-all.c    | 74 ++++++++++++++++++++++++++++++++++++++++--
 accel/kvm/trace-events |  1 +
 hw/intc/ioapic.c       | 11 +++++--
 include/sysemu/kvm.h   |  4 +++
 4 files changed, 86 insertions(+), 4 deletions(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index d49b74512a..b766b6e93c 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -159,9 +159,62 @@ static const KVMCapabilityInfo kvm_required_capabilites[] 
= {
 static NotifierList kvm_irqchip_change_notifiers =
     NOTIFIER_LIST_INITIALIZER(kvm_irqchip_change_notifiers);
 
+struct KVMResampleFd {
+    int gsi;
+    EventNotifier *resample_event;
+    QLIST_ENTRY(KVMResampleFd) node;
+};
+typedef struct KVMResampleFd KVMResampleFd;
+
+/*
+ * Only used with split irqchip where we need to do the resample fd
+ * kick for the kernel from userspace.
+ */
+static QLIST_HEAD(, KVMResampleFd) kvm_resample_fd_list =
+    QLIST_HEAD_INITIALIZER(kvm_resample_fd_list);
+
 #define kvm_slots_lock(kml)      qemu_mutex_lock(&(kml)->slots_lock)
 #define kvm_slots_unlock(kml)    qemu_mutex_unlock(&(kml)->slots_lock)
 
+static inline void kvm_resample_fd_remove(int gsi)
+{
+    KVMResampleFd *rfd;
+
+    QLIST_FOREACH(rfd, &kvm_resample_fd_list, node) {
+        if (rfd->gsi == gsi) {
+            QLIST_REMOVE(rfd, node);
+            break;
+        }
+    }
+}
+
+static inline void kvm_resample_fd_insert(int gsi, EventNotifier *event)
+{
+    KVMResampleFd *rfd = g_new0(KVMResampleFd, 1);
+
+    rfd->gsi = gsi;
+    rfd->resample_event = event;
+
+    QLIST_INSERT_HEAD(&kvm_resample_fd_list, rfd, node);
+}
+
+void kvm_resample_fd_notify(int gsi)
+{
+    KVMResampleFd *rfd;
+
+    if (!kvm_irqchip_is_split()) {
+        return;
+    }
+
+    QLIST_FOREACH(rfd, &kvm_resample_fd_list, node) {
+        if (rfd->gsi == gsi) {
+            event_notifier_set(rfd->resample_event);
+            trace_kvm_resample_fd_notify(gsi);
+            break;
+        }
+    }
+}
+
 int kvm_get_max_memslots(void)
 {
     KVMState *s = KVM_STATE(current_accel());
@@ -1642,8 +1695,25 @@ static int kvm_irqchip_assign_irqfd(KVMState *s, 
EventNotifier *event,
     };
 
     if (rfd != -1) {
-        irqfd.flags |= KVM_IRQFD_FLAG_RESAMPLE;
-        irqfd.resamplefd = rfd;
+        assert(assign);
+        if (kvm_irqchip_is_split()) {
+            /*
+             * When the slow irqchip (e.g. IOAPIC) is in the
+             * userspace, resamplefd will not work because the EOI of
+             * the interrupt will be delivered to userspace instead,
+             * the KVM resample fd kick is skipped.  The userspace
+             * needs to remember the resamplefd and kick it when we
+             * receive EOI of this IRQ.
+             */
+            kvm_resample_fd_insert(virq, resample);
+        } else {
+            irqfd.flags |= KVM_IRQFD_FLAG_RESAMPLE;
+            irqfd.resamplefd = rfd;
+        }
+    } else if (!assign) {
+        if (kvm_irqchip_is_split()) {
+            kvm_resample_fd_remove(virq);
+        }
     }
 
     if (!kvm_irqfds_enabled()) {
diff --git a/accel/kvm/trace-events b/accel/kvm/trace-events
index 4fb6e59d19..a68eb66534 100644
--- a/accel/kvm/trace-events
+++ b/accel/kvm/trace-events
@@ -16,4 +16,5 @@ kvm_set_ioeventfd_mmio(int fd, uint64_t addr, uint32_t val, 
bool assign, uint32_
 kvm_set_ioeventfd_pio(int fd, uint16_t addr, uint32_t val, bool assign, 
uint32_t size, bool datamatch) "fd: %d @0x%x val=0x%x assign: %d size: %d 
match: %d"
 kvm_set_user_memory(uint32_t slot, uint32_t flags, uint64_t guest_phys_addr, 
uint64_t memory_size, uint64_t userspace_addr, int ret) "Slot#%d flags=0x%x 
gpa=0x%"PRIx64 " size=0x%"PRIx64 " ua=0x%"PRIx64 " ret=%d"
 kvm_clear_dirty_log(uint32_t slot, uint64_t start, uint32_t size) 
"slot#%"PRId32" start 0x%"PRIx64" size 0x%"PRIx32
+kvm_resample_fd_notify(int gsi) "gsi %d"
 
diff --git a/hw/intc/ioapic.c b/hw/intc/ioapic.c
index 15747fe2c2..8c75465c62 100644
--- a/hw/intc/ioapic.c
+++ b/hw/intc/ioapic.c
@@ -236,8 +236,15 @@ void ioapic_eoi_broadcast(int vector)
         for (n = 0; n < IOAPIC_NUM_PINS; n++) {
             entry = s->ioredtbl[n];
 
-            if ((entry & IOAPIC_VECTOR_MASK) != vector ||
-                ((entry >> IOAPIC_LVT_TRIGGER_MODE_SHIFT) & 1) != 
IOAPIC_TRIGGER_LEVEL) {
+            if ((entry & IOAPIC_VECTOR_MASK) != vector) {
+                continue;
+            }
+
+            /* Kick resamplefd if KVM is bypassed */
+            kvm_resample_fd_notify(n);
+
+            if (((entry >> IOAPIC_LVT_TRIGGER_MODE_SHIFT) & 1) !=
+                IOAPIC_TRIGGER_LEVEL) {
                 continue;
             }
 
diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
index 141342de98..b67552c047 100644
--- a/include/sysemu/kvm.h
+++ b/include/sysemu/kvm.h
@@ -555,4 +555,8 @@ int kvm_set_one_reg(CPUState *cs, uint64_t id, void 
*source);
 int kvm_get_one_reg(CPUState *cs, uint64_t id, void *target);
 struct ppc_radix_page_info *kvm_get_radix_page_info(void);
 int kvm_get_max_memslots(void);
+
+/* Notify resamplefd for EOI of specific level triggered interrupts */
+void kvm_resample_fd_notify(int gsi);
+
 #endif
-- 
2.24.1