linux-kernel - [RFC 08/14] vfio/nvgrace-egm: Expose EGM region as char device

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Message-ID: <20250904040828.319452-9-ankita@nvidia.com>
Date: Thu, 4 Sep 2025 04:08:22 +0000
From: <ankita@...dia.com>
To: <ankita@...dia.com>, <jgg@...dia.com>, <alex.williamson@...hat.com>,
	<yishaih@...dia.com>, <skolothumtho@...dia.com>, <kevin.tian@...el.com>,
	<yi.l.liu@...el.com>, <zhiw@...dia.com>
CC: <aniketa@...dia.com>, <cjia@...dia.com>, <kwankhede@...dia.com>,
	<targupta@...dia.com>, <vsethi@...dia.com>, <acurrid@...dia.com>,
	<apopple@...dia.com>, <jhubbard@...dia.com>, <danw@...dia.com>,
	<anuaggarwal@...dia.com>, <mochs@...dia.com>, <kjaju@...dia.com>,
	<dnigam@...dia.com>, <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: [RFC 08/14] vfio/nvgrace-egm: Expose EGM region as char device

From: Ankit Agrawal <ankita@...dia.com>

The EGM module expose the various EGM regions as a char device. A
usermode app such as Qemu may mmap to the region and use as VM sysmem.
Each EGM region is represented with a unique char device /dev/egmX
bearing a distinct minor number.

EGM module implements the mmap file_ops to manage the usermode app's
VMA mapping to the EGM region. The appropriate region is determined
from the minor number.

Note that the EGM memory region is invisible to the host kernel as it
is not present in the host EFI map. The host Linux MM thus cannot manage
the memory, even though it is accessible on the host SPA. The EGM module
thus use remap_pfn_range() to perform the VMA mapping to the EGM region.

Suggested-by: Aniket Agashe <aniketa@...dia.com>
Signed-off-by: Ankit Agrawal <ankita@...dia.com>
---
 drivers/vfio/pci/nvgrace-gpu/egm.c | 99 ++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)

diff --git a/drivers/vfio/pci/nvgrace-gpu/egm.c b/drivers/vfio/pci/nvgrace-gpu/egm.c
index 12d4e6e83fff..c2dce5fa797a 100644
--- a/drivers/vfio/pci/nvgrace-gpu/egm.c
+++ b/drivers/vfio/pci/nvgrace-gpu/egm.c
@@ -10,15 +10,114 @@
 
 static dev_t dev;
 static struct class *class;
+static DEFINE_XARRAY(egm_chardevs);
+
+struct chardev {
+	struct device device;
+	struct cdev cdev;
+};
+
+static int nvgrace_egm_open(struct inode *inode, struct file *file)
+{
+	return 0;
+}
+
+static int nvgrace_egm_release(struct inode *inode, struct file *file)
+{
+	return 0;
+}
+
+static int nvgrace_egm_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	return 0;
+}
+
+static const struct file_operations file_ops = {
+	.owner = THIS_MODULE,
+	.open = nvgrace_egm_open,
+	.release = nvgrace_egm_release,
+	.mmap = nvgrace_egm_mmap,
+};
+
+static void egm_chardev_release(struct device *dev)
+{
+	struct chardev *egm_chardev = container_of(dev, struct chardev, device);
+
+	kvfree(egm_chardev);
+}
+
+static struct chardev *
+setup_egm_chardev(struct nvgrace_egm_dev *egm_dev)
+{
+	struct chardev *egm_chardev;
+	int ret;
+
+	egm_chardev = kvzalloc(sizeof(*egm_chardev), GFP_KERNEL);
+	if (!egm_chardev)
+		goto create_err;
+
+	device_initialize(&egm_chardev->device);
+
+	/*
+	 * Use the proximity domain number as the device minor
+	 * number. So the EGM corresponding to node X would be
+	 * /dev/egmX.
+	 */
+	egm_chardev->device.devt = MKDEV(MAJOR(dev), egm_dev->egmpxm);
+	egm_chardev->device.class = class;
+	egm_chardev->device.release = egm_chardev_release;
+	egm_chardev->device.parent = &egm_dev->aux_dev.dev;
+	cdev_init(&egm_chardev->cdev, &file_ops);
+	egm_chardev->cdev.owner = THIS_MODULE;
+
+	ret = dev_set_name(&egm_chardev->device, "egm%lld", egm_dev->egmpxm);
+	if (ret)
+		goto error_exit;
+
+	ret = cdev_device_add(&egm_chardev->cdev, &egm_chardev->device);
+	if (ret)
+		goto error_exit;
+
+	return egm_chardev;
+
+error_exit:
+	kvfree(egm_chardev);
+create_err:
+	return NULL;
+}
+
+static void del_egm_chardev(struct chardev *egm_chardev)
+{
+	cdev_device_del(&egm_chardev->cdev, &egm_chardev->device);
+	put_device(&egm_chardev->device);
+}
 
 static int egm_driver_probe(struct auxiliary_device *aux_dev,
 			    const struct auxiliary_device_id *id)
 {
+	struct nvgrace_egm_dev *egm_dev =
+		container_of(aux_dev, struct nvgrace_egm_dev, aux_dev);
+	struct chardev *egm_chardev;
+
+	egm_chardev = setup_egm_chardev(egm_dev);
+	if (!egm_chardev)
+		return -EINVAL;
+
+	xa_store(&egm_chardevs, egm_dev->egmpxm, egm_chardev, GFP_KERNEL);
+
 	return 0;
 }
 
 static void egm_driver_remove(struct auxiliary_device *aux_dev)
 {
+	struct nvgrace_egm_dev *egm_dev =
+		container_of(aux_dev, struct nvgrace_egm_dev, aux_dev);
+	struct chardev *egm_chardev = xa_erase(&egm_chardevs, egm_dev->egmpxm);
+
+	if (!egm_chardev)
+		return;
+
+	del_egm_chardev(egm_chardev);
 }
 
 static const struct auxiliary_device_id egm_id_table[] = {
-- 
2.34.1