Re: [PATCH v4 0/3] rpmsg: Introduce RPMSG_CREATE_EPT_FD_IOCTL uAPI

[Dawei Li <dawei.li@linux.dev>]

Hi Arnaud, 
Thanks for review.

On Wed, Jun 18, 2025 at 03:07:36PM +0200, Arnaud POULIQUEN wrote:
> Hello Dawei,
> 
> 
> Please find a few comments below. It is not clear to me which parts of your
> implementation are mandatory and which are optional "nice-to-have" optimizations.

It's more like an improvement.

> 
> Based on (potentially erroneous) hypothesis, you will find a suggestion for an
> alternative to the anonymous inode approach, which does not seem to be a common
> interface.

AFAIC, annoymous inode is a common interface and used extensivly in kernel development.
Some examples below.

> 
> 
> On 6/9/25 17:15, Dawei Li wrote:
> > Hi,
> > 
> > This is V4 of series which introduce new uAPI(RPMSG_CREATE_EPT_FD_IOCTL)
> > for rpmsg subsystem.
> > 
> > Current uAPI implementation for rpmsg ctrl & char device manipulation is
> > abstracted in procedures below:
> > - fd = open("/dev/rpmsg_ctrlX")
> > - ioctl(fd, RPMSG_CREATE_EPT_IOCTL, &info); /dev/rpmsgY devnode is
> >   generated.
> > - fd_ep = open("/dev/rpmsgY", O_RDWR) 
> > - operations on fd_ep(write, read, poll ioctl)
> > - ioctl(fd_ep, RPMSG_DESTROY_EPT_IOCTL)
> > - close(fd_ep)
> > - close(fd)
> > 
> > This /dev/rpmsgY abstraction is less favorable for:
> > - Performance issue: It's time consuming for some operations are
> > invovled:
> >   - Device node creation.
> >     Depends on specific config, especially CONFIG_DEVTMPFS, the overall
> >     overhead is based on coordination between DEVTMPFS and userspace
> >     tools such as udev and mdev.
> > 
> >   - Extra kernel-space switch cost.
> > 
> >   - Other major costs brought by heavy-weight logic like device_add().
> 
> Is this a blocker of just optimization?

Yep, performance is one of motivations of this change.

> 
> > 
> > - /dev/rpmsgY node can be opened only once. It doesn't make much sense
> >     that a dynamically created device node can be opened only once.
> 
> 
> I assume this is blocker with the fact that you need to open the /dev/rpmsg<x>
> to create the endpoint.

Yes. You have to open /dev/rpmsgX which is generated by legacy ioctl to
instantiate a new endpoint.

> 
> 
> > 
> > - For some container application such as docker, a client can't access
> >   host's dev unless specified explicitly. But in case of /dev/rpmsgY, which
> >   is generated dynamically and whose existence is unknown for clients in
> >   advance, this uAPI based on device node doesn't fit well.
> 
> does this could be solve in userspace parsing /sys/class/rpmsg/ directory to
> retreive the device?

Hardly, because client still can't access /dev/rpmsgX which is generated
by host _after_ client is launched.

> 
> You could face same kind of random instantiation for serial peripherals ( UART;
> USb, I2C,...) based on a device tree enumeration. I suppose that user space
> use to solve this.
> 
> > 
> > An anonymous inode based approach is introduced to address the issues above.
> > Rather than generating device node and opening it, rpmsg code just creates
> > an anonymous inode representing eptdev and return the fd to userspace.
> 
> A drawback is that you need to share fb passed between processes.

Fd is the abstraction of an unique endpoint device, it holds true for
both legacy and new approach.

So I guess what you mean is that /dev/rpmsgX is global to all so other process
can access it?

But /dev/rpmsgX is designed to be opened only once, it's implemented as
singleton pattern.

static int rpmsg_eptdev_open(struct inode *inode, struct file *filp)
{
...
        if (eptdev->ept) {
                mutex_unlock(&eptdev->ept_lock);
                return -EBUSY;
        }
...
        eptdev->ept = ept;
...
}

[...]
 
> > 	printf("loop[%d]\n", loop);
> > 
> > 	gettimeofday(&start, NULL);
> > 
> > 	while (loop--) {
> 
> Do you need to create /close Endpoint sevral times in your real use case with
> high timing
> constraint?

No, it's just a silly benchmark demo, large sample reduces noise statistically.

> 
> > 		fd_info.fd = -1;
> > 		fd_info.flags = O_RDWR | O_CLOEXEC | O_NONBLOCK;
> > 		ret = ioctl(fd, RPMSG_CREATE_EPT_FD_IOCTL, &fd_info);
> > 		if (ret < 0 || fd_info.fd < 0) {
> > 			printf("ioctl[RPMSG_CREATE_EPT_FD_IOCTL] failed, ret[%d]\n", ret);
> > 		}
> > 
> 
> 
> > 		ret = ioctl(fd_info.fd, RPMSG_DESTROY_EPT_IOCTL, &info);
> > 		if (ret < 0) {
> > 			printf("new ioctl[RPMSG_DESTROY_EPT_IOCTL] failed, ret[%d]\n", ret);
> > 		}
> > 
> > 		close(fd_info.fd);
> 
> It seems strange to me to use ioctl() for opening and close() for closing, from
> a symmetry point of view.

Sorry to hear that. But no, it's a pretty normal skill in kernel codebase
, I had to copy some examples from reply to other reviewer[1].

anon_inode_get_{fd,file} are used extensively in kernel for returning a new
fd to userspace which is associated with an unique data structure in kernel
space, in different ways:

- via ioctl(), some examples are:

 - KVM ioctl(s)
   - KVM_CREATE_VCPU -> kvm_vm_ioctl_create_vcpu
   - KVM_GET_STATS_FD -> kvm_vcpu_ioctl_get_stats_fd
   - KVM_CREATE_DEVICE -> kvm_ioctl_create_device
   - KVM_CREATE_VM -> kvm_dev_ioctl_create_vm

 - DMA buf/fence/sync ioctls
   - DMA_BUF_IOCTL_EXPORT_SYNC_FILE -> dma_buf_export_sync_file
   - SW_SYNC_IOC_CREATE_FENCE -> sw_sync_ioctl_create_fence
   - Couples of driver implement DMA buf by using anon file _implicitly_:
     - UDMABUF_CREATE -> udmabuf_ioctl_create
     - DMA_HEAP_IOCTL_ALLOC -> dma_heap_ioctl_allocate

 - gpiolib ioctls:
   - GPIO_GET_LINEHANDLE_IOCTL -> linehandle_create
   - GPIO_V2_GET_LINE_IOCTL

 -  IOMMUFD ioctls:

 -  VFIO Ioctls:

 - ....


- via other specific syscalls:
 - epoll_create1
 - bpf
 - perf_event_open
 - inotify_init
 - ...

[1] https://lore.kernel.org/all/20250530125008.GA5355@wendao-VirtualBox/

> 
> Regarding your implementation, I wonder if we could keep the /dev/rpmsg<x>
> device with specific open() and close() file operations associated with your new
> ioctl.
> 
> - The ioctl would create the endpoint.
> - The open() and close() operations would simply manage the file descriptor and
> increment/decrement a counter to prevent premature endpoint destruction.
> 
> 
> Regards,
> Arnaud
> 

[...]

Thanks,

	Dawei