x86/hyperv: setup hypercall page

x86/hyperv: setup hypercall page

Hyper-V uses a technique called overlay page for its hypercall page. It
will insert a backing page to the guest when the hypercall functionality
is enabled. That means we can use a page that is not backed by real
memory for hypercall page.

To avoid shattering L0 superpages and treading on any MMIO areas
residing in low addresses, use the top-most addressable page for that
purpose. Adjust e820 map accordingly.

We also need to register Xen's guest OS ID to Hyper-V. Use 0x3 as the
vendor ID. Fix the comment in hyperv-tlfs.h while at it.

Signed-off-by: Wei Liu <liuwe@microsoft.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Paul Durrant <pdurrant@amazon.com>