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SubjectRE: [RFC, PATCH 5/24] i386 Vmi code patching
> -----Original Message-----
> From:
> Sent: Wednesday, March 22, 2006 5:34 PM
> > The idea of in-tree ROM code doesn't make sense. The entire point
> > of this layer of code is that it is modular, and specific to the
> > hypervisor, not the kernel. Once you lift the shroud and combine
> > the two layers, you have lost all of the benefit that it was
> > supposed to provide.
> To elaborate a bit more, the "ROM" layer is "published" by
> the hypervisor. This layer of abstraction will let you take
> a VMI-compiled kernel and run it efficiently on any
> hypervisor that exports a VMI interface - even one that you
> didn't know about (or didn't exist) when you compiled your kernel.
> [...]
> Going forward, having the ROM layer published by the
> hypervisor gives the hypervisor more flexibility than having
> the code statically compiled into the kernel. Consider when
> hardware virtualization becomes more prevalent. Perhaps
> there are places where today hypercalls make sense, but with
> hardware virtualization, you'd rather have the hardware just
> take care of it. CPUID is the only example I can come up
> with at the moment, but there are certainly others. VMI lets
> the hypervisor decide that it doesn't actually need to
> replace the CPUID instruction with a hypercall. The
> important factor here is that only the hypervisor, not the
> kernel, knows about these performance tradeoffs.

Very obvious other candidates are the shadowed system state registers
(cli, sti, CRx) provided by VT and the shadow page-table support as
defined by Pacifica. In particular since these features are dependent on
the specific processor revision a hard-coded binary interface doesn't do
any good. The ROM pretty much resembles Linux' system call interface as
provided today optimizing for the specific HW architecture.

- Volkmar
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