Re: [PATCH] reserve end-of-conventional-memory to 1MB on 32-bit v2

H. Peter Anvin
2008-03-04T17:10:19Z

Alexander van Heukelum wrote:
>
> If this is indeed not executed, is there a way to detect whether
> we can expect the environment to behave like a normal pc in terms
> of magic addresses, bios areas, isa reserved address space and so
> on?
>

The subarch stuff (boot_params.hdr.hardware_subarch) is indeed executed,
at least with newer PV guests.

However, as far as this kind of stuff, one really have to wonder to what
extent the PV users really care about how much they perturb the guests.
The canonical view of virtualization is that you should perturb your
guests as little as possible, but that doesn't really seem to be
considered in the observable bits of the PV universe as far as I can tell.

A *massive* issue with hooks -- including paravirt_ops -- is that they
are largely undocumented both in code and in specification, and usually
hard-code the underlying implemenentation at a specific point in time: I
have yet to see any sort of specification document for paravirt_ops, and
most of the hooks are simply empty on hardware. This means that the
burden has shifted onto the kernel maintainers to test every possible
paravirt_ops client, because it is quite literally the only way to know
when it's broken.

I'm starting to feel that the PV clients need to document their
environments and their constraints better for the benefit of the core
maintainers.

-hpa