[Xen-devel] vpci: deferral of register write until p2m changes are done

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

Hello,

While doing the recent vPCI fixes and also working on SR-IOV support
I've been thinking about how vPCI handles writes to PCI registers that
imply modifications to the p2m for PVH Dom0.

When memory decoding or ROM BARs are enabled Xen performs the
following flow:

1. Create a rangeset with the memory regions that need to be
mapped/unmapped.
2. Block the vCPU and perform the p2m changes in a preemptive way.
3. After the p2m changes have been applied (or in case of error) write
to the register in order to enable/disable memory decoding or the ROM
BAR and mark the BARs as enabled.

I'm unsure about the benefit of deferring the register write (step 3)
for a PVH Dom0, so I would like to perform the register write before
applying the changes to the p2m.

This would simplify the logic of the deferred p2m operations by not
having to store the register value, and would also remove the ugly
hack that we currently have for ROM BARs where vPCI fabricates a dummy
command register just to signal whether the operation is a map or
unmap.

Thanks, Roger.

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