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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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