Hi, I was trying to use VFIO to do IO and use MSIX interrupts, however I noticed that it is not supported to have multiple users. So I cannot have process A use IO region and process B use a different IO region because VFIO group/container/device cannot be opened by 2 processes. (It is possible to do a fork yes... but not 2 separate launched processes right?)

There is a comment in vfio_group_fops_open: /* Do we need multiple instances of the group open? Seems not. */

So no one else requires this functionality yet? Or has anyone looked into this use case already?

Thanks,

Armando Ruiz

What is the use case? Are you running two different VMs at the same time?

Using two VMs at the same time is indeed one use case for this situation. The generic use case is as follows: Process A (VM-A) does not know that Process B(VM-B) is running so it could or could not be using the device already. Process A should be able to use the same device than Process B as long as the IO region is different (can it even support using the same IO region?). I am currently not using a VM for this, I just have my own application which opens a device PCIe BAR 0 and maps offset X, and try to open a second application which also opens PCIe BAR0 and maps offset Y. The second application fails to open vfio as it is already opened.

Won't changing the BAR break the first process, since it changes where the device memory is mapped?

It took me some time to check this but if I am looking correctly at vfio_group_get_device_fd, if the device is already opened it is reused not opened again so the bar is mapped once and can be reused by more than one "user". However, it seem like this extra user is limited to the same process because still it will not be possible to open the group from another process. So again I think this function vfio_group_fops_open is the one limiting this use case right?