linuxkpi: Add eventfd_*()

Add <linux/eventfd.h> and expose the `eventfd_*()` API. This is used by
DRM drivers for some time, but the code was commented out so far.

Note that Linux uses `struct eventfd_ctx`, but FreeBSD defines `struct
eventfd`. We define `eventfd_ctx` as a synonym to `eventfd`.

Reviewed by:	christos, markj
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50853

sys/compat/linuxkpi/common/include/linux/eventfd.h

@@ -0,0 +1,54 @@
+/*-
+ * Copyright (c) 2025 The FreeBSD Foundation
+ * Copyright (c) 2025 Jean-Sébastien Pédron
+ *
+ * This software was developed by Jean-Sébastien Pédron under sponsorship
+ * from the FreeBSD Foundation.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef	_LINUXKPI_LINUX_EVENTFD_H_
+#define	_LINUXKPI_LINUX_EVENTFD_H_
+
+#include <sys/eventfd.h>
+
+#include <linux/wait.h>
+#include <linux/err.h>
+#include <linux/percpu-defs.h>
+#include <linux/percpu.h>
+#include <linux/sched.h>
+
+/*
+ * Linux uses `struct eventfd_ctx`, but FreeBSD defines `struct eventfd`. Here,
+ * we define a synonym to the FreeBSD structure. This allows to keep Linux code
+ * unmodified.
+ */
+#define	eventfd_ctx eventfd
+
+#define	eventfd_ctx_fdget lkpi_eventfd_ctx_fdget
+struct eventfd_ctx *lkpi_eventfd_ctx_fdget(int fd);
+
+#define	eventfd_ctx_put lkpi_eventfd_ctx_put
+void lkpi_eventfd_ctx_put(struct eventfd_ctx *ctx);
+
+#endif /* _LINUXKPI_LINUX_EVENTFD_H_ */

sys/compat/linuxkpi/common/src/linux_eventfd.c

@@ -0,0 +1,63 @@
+/*-
+ * Copyright (c) 2025 The FreeBSD Foundation
+ * Copyright (c) 2025 Jean-Sébastien Pédron
+ *
+ * This software was developed by Jean-Sébastien Pédron under sponsorship
+ * from the FreeBSD Foundation.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/param.h>
+#include <sys/file.h>
+#include <sys/filedesc.h>
+
+#include <linux/eventfd.h>
+
+struct eventfd_ctx *
+lkpi_eventfd_ctx_fdget(int fd)
+{
+	struct file *fp;
+	struct eventfd_ctx *ctx;
+
+	/* Lookup file pointer by file descriptor index. */
+	if (fget_unlocked(curthread, fd, &cap_no_rights, &fp) != 0)
+		return (ERR_PTR(-EBADF));
+
+	/*
+	 * eventfd_get() bumps the refcount, so we can safely release the
+	 * reference on the file itself afterwards.
+	 */
+	ctx = eventfd_get(fp);
+	fdrop(fp, curthread);
+
+	if (ctx == NULL)
+		return (ERR_PTR(-EBADF));
+
+	return (ctx);
+}
+
+void
+lkpi_eventfd_ctx_put(struct eventfd_ctx *ctx)
+{
+	eventfd_put(ctx);
+}

sys/conf/files

@@ -4662,6 +4662,8 @@ compat/linuxkpi/common/src/linux_dmi.c		optional compat_linuxkpi \
 	compile-with "${LINUXKPI_C}"
 compat/linuxkpi/common/src/linux_domain.c	optional compat_linuxkpi \
 	compile-with "${LINUXKPI_C}"
+compat/linuxkpi/common/src/linux_eventfd.c	optional compat_linuxkpi \
+	compile-with "${LINUXKPI_C}"
 compat/linuxkpi/common/src/linux_firmware.c	optional compat_linuxkpi \
 	compile-with "${LINUXKPI_C}"
 compat/linuxkpi/common/src/linux_folio.c	optional compat_linuxkpi \

sys/modules/linuxkpi/Makefile

@@ -6,6 +6,7 @@ SRCS=	linux_compat.c \
 	linux_devres.c \
 	linux_dmi.c \
 	linux_domain.c \
+	linux_eventfd.c \
 	linux_firmware.c \
 	linux_folio.c \
 	linux_fpu.c \