xen/arm: vgic: Optimize the way to store the target vCPU in the rank

xen/arm: vgic: Optimize the way to store the target vCPU in the rank

Xen is currently directly storing the value of GICD_ITARGETSR register
(for GICv2) and GICD_IROUTER (for GICv3) in the rank. This makes the
emulation of the registers access very simple but makes the code to get
the target vCPU for a given vIRQ more complex.

While the target vCPU of an vIRQ is retrieved every time an vIRQ is
injected to the guest, the access to the register occurs less often.

So the data structure should be optimized for the most common case
rather than the inverse.

This patch introduces the usage of an array to store the target vCPU for
every interrupt in the rank. This will make the code to get the target
very quick. The emulation code will now have to generate the GICD_ITARGETSR
and GICD_IROUTER register for read access and split it to store in a
convenient way.

With the new way to store the target vCPU, the structure vgic_irq_rank
is shrunk down from 320 bytes to 92 bytes. This is saving about 228
bytes of memory allocated separately per vCPU.

Note that with these changes, any read to those register will list only
the target vCPU used by Xen. As the spec is not clear whether this is a
valid choice or not, OSes which have a different interpretation of the
spec (i.e OSes which perform read-modify-write operations on these
registers) may not boot anymore on Xen. Although, I think this is fair
trade between memory usage in Xen (1KB less on a domain using 4 vCPUs
with no SPIs) and a strict interpretation of the spec (though all the
cases are not clearly defined).

Furthermore, the implementation of the callback get_target_vcpu is now
exactly the same. Consolidate the implementation in the common vGIC code
and drop the callback.

Finally take the opportunity to fix coding style and replace "irq" by
"virq" to make clear that we are dealing with virtual IRQ in section we
are modifying.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>