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Re: [Xen-devel] Xen's Linux kernel config options

On Fri, Dec 12, 2014 at 9:29 AM, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> On 12/12/14 13:17, Juergen Gross wrote:
> Move XEN_PVHVM under XEN and have it select PARAVIRT and PARAVIRT_CLOCK.

FWIW, although it seems we do not want to let users just build
XEN_PVHVM hypervisors I have the changes required now to at least get
this to build so I do know what it takes.

>> XEN_FRONTEND                                            XEN_PV ||
>>                                                         XEN_PVH ||
>>                                                         XEN_PVHVM
> This enables all the basic infrastructure for frontends: event channels,
> grant tables and Xenbus.
> Don't make XEN_FRONTEND depend on any XEN_* variant.  It should be
> possible to have frontend drivers without support for any of the
> PV/PVHVM/PVH guest types.

David, can you elaborate on the type of Xen guest it would be on x86
its not PV, PVHVM, or PVH? I'm particularly curious about the
xen_domain_type and how it would end up to selected. As it is we tie
in XEN_PVHVM at build time with XEN_PVH, in order to have XEN_PVHVM
completely removed from XEN_PVH we need quite a bit of code changes
which at least as code exercise I have completed already. If we want
at the very least xen_domain_type set when XEN_PV, XEN_PVHVM, and
XEN_PVH are not available we need a bit more work.

>  Frontends only need event channels, grant
> table and xenbus.

Well xenbus_probe_initcall() will check for xen_domain() and that
won't be set on x86 right now unless we have XEN_PV, XEN_PVHVM or
XEN_PVH set -- to start off with. Then
drivers/xen/xenbus/xenbus_client.c will check xen_feature in quite a
bit of places as well, that won't be set unless xen_setup_features()
is called which right now is only done on x86 arch/x86/xen/enlighten.c
which as Juergen pointed out, is not needed if you don't have XEN_PV
or XEN_PVH. As it turns out this is incorrect though, its needed for
XEN_PVHVM as well and my split exercise in code addresses this. Now,
at least in my code if you don't have XEN_PV, XEN_PVHVM, or XEN_PVH we
don't call xen_setup_features() and its unclear to me where or how
that should happen in other cases.

> Perhaps have XEN_FRONTEND select XEN instead?

Right now if you enable CONFIG_XEN xen-head.S brings in and assumes a
big tamale of guest support on x86, there are quite a bit of other
code that also relies on CONFIG_XEN for similar purposes, and trying
to remove out XEN_FRONTEND dependency from XEN_PV, XEN_PVHVM, XEN_PVH
requires quite a bit work, most of which I think I've done, the only
puzzle remaining to me at least is what we want to do for the setup
for non XEN_PV, XEN_PVHVM, XEN_PVH Linux systems.


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