[Xen-devel] Re: XEN/IA64 VHPT support

[Dan Magenheimer <dan.magenheimer@xxxxxx>]

>       Probably you can rely on us for the long format VHPT although
> the detail plan is not done yet. BTW, for the hypercall, I am thinking
> how it will be implemented in IA64 architecture. I think a straight
> forward way is to use break instruction like linux system call, any idea
> in your mind? 

Re LVHPT.  Please let me know if you work on it as it has been
something I've been wanting to get done for some time, mostly because
it will make simulations run faster too.  So if you don't get to
it soon, I might work on it, and it would be bad if we both
finished at the same time :-)

Re hypercalls.  Actually there are hypercalls already in Xen/ia64
and they do use the break instruction.  It's just that the domain
calls them indirectly so perhaps they are better called "hyperthunks"
("Thunk" is a rarely used compiler term which originated in Algol 60
that means: "code that is used to convert calls to one interface
into calls to another"):

When a domain is created, fake EFI/SAL/PAL (firmware) calls are created
that contain the hypercalls.  Thus when domain0 invokes an EFI/SAL/PAL
call, it calls a "stub" routine that invokes a break instruction/fault
that is recognized in Xen/ia64 as a hypercall, with arguments passed
in general registers (unlike Xen/x86 which passes hypercall arguments
in memory).

You may want to step through a domain0 firmware call to see this work.
If I recall correctly, the code that creates the hyperthunks is
in arch/ia64/dom_fw.c

While my intent is to use the same mechanism (breaks) for "real" hypercalls,
I also want to explore the use of fast system calls (recently added
in Linux/ia64 by David Mosberger) using the epc instruction in certain
cases.

Dan




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