[DISCUSSION] Reworking remote controller access

[Ganesh G R <ganeshgr@linux.ibm.com>]

Hi all,

A few days ago during the review of the P11 patches, Cedric suggested 
removing the use of qdev_get_machine() 
[https://lore.kernel.org/qemu-devel/3456f764-616b-412c-839c-aaef4bf1e47c@redhat.com/] 
and instead using MMIO to obtain the address of remote virtualization 
structures. However, it wasn't feasible to implement this change 
alongside the P11 updates, so we opted for a temporary workaround to 
eliminate the use of qdev_get_machine().
[https://lore.kernel.org/qemu-devel/900db31c-54d5-410d-934e-347ca0b22ec2@redhat.com/]

Now, we are planning to revisit and improve the way we access remote 
controllers. We have considered a couple of approaches:

1. Implement MMIO operations for all BARs
Redirect all remote access through the BARs instead of iterating over 
all XIVE instances to retrieve the virtualization structure addresses.

2. Use a dedicated MMIO offset to return the address of the 
virtualization structure:
This is a simpler solution, similar to what Cedric implemented for P9, 
the idea is to define a special MMIO offset that when accessed returns 
the address directly. This avoids the need for qdev_get_machine() and 
keeps the design simple.

Any suggestions or feedback on these approaches.

Thanks
Ganesh