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    SubjectRe: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable
    Ingo Molnar wrote:

    > * David Miller <davem@davemloft.net> wrote:

    > > Perhaps the problem can be dealt with using ELF relocations.
    > >
    > > There is another case, discussed yesterday on netdev, where run-time
    > > resolution of ELF relocations would be useful (for
    > > very-very-very-read-only variables) so if it can solve this problem
    > > too it would be nice to have a generic infrastructure for it.
    >


    > yeah, and i really think this is very fundamental: [...]

    I think what Dave is suggesting is that we use the reloc information the
    compiler generates to find the patchable callsites rather than have
    special wrappers. This is an interesting idea.

    > Limited, instruction-level patching like alternatives.h is fine because
    > that makes it easier to support multiple, incompatible CPU
    > architectures, without having to do a hugely intrusive split at the
    > kernel RPM level.
    >
    > but the level of 'binary patching' done by the paravirt and Xen goes way
    > beyond that,

    Not really. There are only three cases:

    1. replace an indirect call with a direct call
    2. nop out a callsite
    3. patch in a short inline sequence

    And as I pointed out, this is used by all pv_op backends, using a common
    piece of code to implement at least 1 and 2. 3 could be implemented
    semi-generically by using rules like "if (func == native_sti) {
    patch("sti"); }", which would cover many cases where a hypervisor
    doesn't need any special handling for a particular operation.

    The goal is to eliminate the cost of the indirect calls with nice
    predictable indirect calls. There's a 1 byte/callsite overhead, but I
    don't think that's a horrible overhead.

    And, at worst, its only a little more complex than the kinds of
    transformations.

    Ideally, its a mechanism which could be used elsewhere. It applies with
    you have some kind of ops_vector table which is updated once (or perhaps
    very rarely), and you don't want to wear the overhead of indirect calls
    everywhere.

    > and the changes here really underscore that we:
    >
    > _should not emulate the closed source world_
    >
    > There the only solution is to binary-patch - because they have no source
    > code. But here, we've got all the source code.
    >

    I don't think this is a relevant comparison. This is purely a matter of
    optimising out unnecessary indirect calls.

    > nobody wants to boot a xen-paravirt kernel from a floppy, so image size
    > is not an issue. In-RAM overhead would in fact be /reduced/, because
    > currently all the paravirt overhead hits both the native and the
    > paravirt kernel. Nor would /all/ of the vmlinuz have to be replicated in
    > the images - it's enough to replicate only those functions that truly
    > differ between the two build methods.

    One of the explicit goals of pv_ops was to allow a single kernel to
    either boot on native hardware or under any one of the supported
    hypervisors, explicitly to avoid having to manage multiple kernel
    images. Compiling the kernel N+1 times for N hypervisors, and then
    bundling them up in some kind of multi-image format doesn't seem like a
    particularly good tradeoff. The kernel RPM on my machine here is
    already ~50Mbytes; expanding that to 250Mbytes to support native, Xen,
    vmi, lguest and kvm doesn't seem reasonable.

    J
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