Re: [Xen-devel] [PATCH v2 2/2] docs: Introduce some hypercall page documentation

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.05.19 at 06:10, <andrew.cooper3@xxxxxxxxxx> wrote:
> This also introduced the top-level Guest Documentation section.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
with one further remark / question:

> +Hypercall Page
> +==============
> +
> +The hypercall page is a page of guest RAM into which Xen will write suitable
> +transfer stubs.
> +
> +Creating a hypercall page is an isolated operation from Xen's point of view.
> +It is the guests responsibility to ensure that the hypercall page, once
> +written by Xen, is mapped with executable permissions so it may be used.
> +Multiple hypercall pages may be created by the guest, if it wishes.
> +
> +The stubs are arranged by hypercall index, and start on 32-byte boundaries.
> +To invoke a specific hypercall, ``call`` the relevant stub [3]_:
> +
> +.. code-block:: none
> +
> +  call hypercall_page + index * 32
> +
> +There result is an ABI which is invariant of the exact operating mode or
> +hardware vendor.  This is intended to simplify guest kernel interfaces by
> +abstracting away the details of how it is currently running.

Is it worth mentioning here that certain aspects of the hypervisor interface
(shared data structures) are influenced by the mode the guest is in at the
time the most recent hypercall page registration (oddly enough alternatively
the most recent setting of HVM_PARAM_CALLBACK_IRQ) has occurred?

Jan



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