[PATCH] ioeventfd: Introduce KVM_IOEVENTFD_FLAG_PIPE

[Sasha Levin <levinsasha928@gmail.com>]

The new flag allows passing a write side of a pipe instead of an
eventfd to be notified of writes to the specified memory region.

Instead of signaling an event, the value written to the memory region
is written to the pipe.

Using a pipe instead of an eventfd is usefull when any value can be
written to the memory region but we're interested in recieving the
actual value instead of just a notification.

A simple example for practical use is the serial port. we are not
interested in an exit every time a char is written to the port, but
we do need to know what was written so we could handle it on the guest.

Cc: Avi Kivity <avi@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
---
 include/linux/kvm.h |    2 +
 virt/kvm/eventfd.c  |   65 +++++++++++++++++++++++++++++++++++---------------
 2 files changed, 47 insertions(+), 20 deletions(-)

diff --git a/include/linux/kvm.h b/include/linux/kvm.h
index 55ef181..548f23a 100644
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -387,12 +387,14 @@ enum {
 	kvm_ioeventfd_flag_nr_datamatch,
 	kvm_ioeventfd_flag_nr_pio,
 	kvm_ioeventfd_flag_nr_deassign,
+	kvm_ioeventfd_flag_nr_pipe,
 	kvm_ioeventfd_flag_nr_max,
 };
 
 #define KVM_IOEVENTFD_FLAG_DATAMATCH (1 << kvm_ioeventfd_flag_nr_datamatch)
 #define KVM_IOEVENTFD_FLAG_PIO       (1 << kvm_ioeventfd_flag_nr_pio)
 #define KVM_IOEVENTFD_FLAG_DEASSIGN  (1 << kvm_ioeventfd_flag_nr_deassign)
+#define KVM_IOEVENTFD_FLAG_PIPE      (1 << kvm_ioeventfd_flag_nr_pipe)
 
 #define KVM_IOEVENTFD_VALID_FLAG_MASK  ((1 << kvm_ioeventfd_flag_nr_max) - 1)
 
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 73358d2..434293e 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -413,10 +413,11 @@ module_exit(irqfd_module_exit);
 
 /*
  * --------------------------------------------------------------------
- * ioeventfd: translate a PIO/MMIO memory write to an eventfd signal.
+ * ioeventfd: translate a PIO/MMIO memory write to an eventfd signal or
+ *            a pipe write.
  *
- * userspace can register a PIO/MMIO address with an eventfd for receiving
- * notification when the memory has been touched.
+ * userspace can register a PIO/MMIO address with an eventfd or a 
+ * pipe for receiving notification when the memory has been touched.
  * --------------------------------------------------------------------
  */
 
@@ -424,6 +425,7 @@ struct _ioeventfd {
 	struct list_head     list;
 	u64                  addr;
 	int                  length;
+	struct file         *pipe;
 	struct eventfd_ctx  *eventfd;
 	u64                  datamatch;
 	struct kvm_io_device dev;
@@ -439,7 +441,11 @@ to_ioeventfd(struct kvm_io_device *dev)
 static void
 ioeventfd_release(struct _ioeventfd *p)
 {
-	eventfd_ctx_put(p->eventfd);
+	if (p->eventfd)
+		eventfd_ctx_put(p->eventfd);
+	else
+		fput(p->pipe);
+
 	list_del(&p->list);
 	kfree(p);
 }
@@ -481,6 +487,21 @@ ioeventfd_in_range(struct _ioeventfd *p, gpa_t addr, int len, const void *val)
 	return _val == p->datamatch ? true : false;
 }
 
+static ssize_t kernel_write(struct file *file, const char *buf, size_t count,
+			    loff_t pos)
+{
+	mm_segment_t old_fs;
+	ssize_t res;
+
+	old_fs = get_fs();
+	set_fs(get_ds());
+	/* The cast to a user pointer is valid due to the set_fs() */
+	res = vfs_write(file, (const char __user *)buf, count, &pos);
+	set_fs(old_fs);
+
+	return res;
+}
+
 /* MMIO/PIO writes trigger an event if the addr/val match */
 static int
 ioeventfd_write(struct kvm_io_device *this, gpa_t addr, int len,
@@ -491,7 +512,11 @@ ioeventfd_write(struct kvm_io_device *this, gpa_t addr, int len,
 	if (!ioeventfd_in_range(p, addr, len, val))
 		return -EOPNOTSUPP;
 
-	eventfd_signal(p->eventfd, 1);
+	if (p->pipe)
+		kernel_write(p->pipe, val, len, 0);
+	else
+		eventfd_signal(p->eventfd, 1);
+
 	return 0;
 }
 
@@ -533,7 +558,7 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 	int                       pio = args->flags & KVM_IOEVENTFD_FLAG_PIO;
 	enum kvm_bus              bus_idx = pio ? KVM_PIO_BUS : KVM_MMIO_BUS;
 	struct _ioeventfd        *p;
-	struct eventfd_ctx       *eventfd;
+	struct eventfd_ctx       *eventfd = NULL;
 	int                       ret;
 
 	/* must be natural-word sized */
@@ -555,9 +580,11 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 	if (args->flags & ~KVM_IOEVENTFD_VALID_FLAG_MASK)
 		return -EINVAL;
 
-	eventfd = eventfd_ctx_fdget(args->fd);
-	if (IS_ERR(eventfd))
-		return PTR_ERR(eventfd);
+	if (!(args->flags & KVM_IOEVENTFD_FLAG_PIPE)) {
+		eventfd = eventfd_ctx_fdget(args->fd);
+		if (IS_ERR(eventfd))
+			return PTR_ERR(eventfd);
+	}
 
 	p = kzalloc(sizeof(*p), GFP_KERNEL);
 	if (!p) {
@@ -568,7 +595,11 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 	INIT_LIST_HEAD(&p->list);
 	p->addr    = args->addr;
 	p->length  = args->len;
-	p->eventfd = eventfd;
+
+	if (args->flags & KVM_IOEVENTFD_FLAG_PIPE)
+		p->pipe = fget(args->fd);
+	else
+		p->eventfd = eventfd;
 
 	/* The datamatch feature is optional, otherwise this is a wildcard */
 	if (args->flags & KVM_IOEVENTFD_FLAG_DATAMATCH)
@@ -601,7 +632,9 @@ unlock_fail:
 
 fail:
 	kfree(p);
-	eventfd_ctx_put(eventfd);
+
+	if (!(args->flags & KVM_IOEVENTFD_FLAG_PIPE))
+		eventfd_ctx_put(eventfd);
 
 	return ret;
 }
@@ -612,20 +645,14 @@ kvm_deassign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 	int                       pio = args->flags & KVM_IOEVENTFD_FLAG_PIO;
 	enum kvm_bus              bus_idx = pio ? KVM_PIO_BUS : KVM_MMIO_BUS;
 	struct _ioeventfd        *p, *tmp;
-	struct eventfd_ctx       *eventfd;
 	int                       ret = -ENOENT;
 
-	eventfd = eventfd_ctx_fdget(args->fd);
-	if (IS_ERR(eventfd))
-		return PTR_ERR(eventfd);
-
 	mutex_lock(&kvm->slots_lock);
 
 	list_for_each_entry_safe(p, tmp, &kvm->ioeventfds, list) {
 		bool wildcard = !(args->flags & KVM_IOEVENTFD_FLAG_DATAMATCH);
 
-		if (p->eventfd != eventfd  ||
-		    p->addr != args->addr  ||
+		if (p->addr != args->addr  ||
 		    p->length != args->len ||
 		    p->wildcard != wildcard)
 			continue;
@@ -641,8 +668,6 @@ kvm_deassign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 
 	mutex_unlock(&kvm->slots_lock);
 
-	eventfd_ctx_put(eventfd);
-
 	return ret;
 }
 
-- 
1.7.6