Bits 63-48 of the I/OAPIC Redirection Table Entry map directly to bits 19-4
of the address used in the resulting MSI cycle.

Historically, the x86 MSI format only used the top 8 of those 16 bits as
the destination APIC ID, and the "Extended Destination ID" in the lower 8
bits was unused.

With interrupt remapping, the lowest bit of the Extended Destination ID
(bit 48 of RTE, bit 4 of MSI address) is now used to indicate a remappable
format MSI.

A hypervisor can use the other 7 bits of the Extended Destination ID to
permit guests to address up to 15 bits of APIC IDs, thus allowing 32768
vCPUs before having to expose a vIOMMU and interrupt remapping to the
guest.

No behavioural change in this patch, since nothing yet permits APIC IDs
above 255 to be used with the non-IR I/OAPIC domain.

[ tglx: Converted it to the cleaned up entry/msi_msg format and added
  	commentry ]
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201024213535.443185-32-dwmw2@infradead.org