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

On Mon, 23 Feb 2015, Luis R. Rodriguez wrote:
> On Thu, Feb 19, 2015 at 3:43 PM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote:
> > 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.
> OK I think I see the issue. We have nothing quite like
> xen_guest_init() on x86 enlighten.c, we do have this for ARM and I
> think I can that close the gap I'm observing.
> >>  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.
> Yeah I think having an x86 equivalent of xen_guest_init() would solve
> this, Stefano, thoughts?

Having xen_guest_init() on x86 would be nice.  Being able to set
xen_domain_type to XEN_HVM_DOMAIN if we are running on Xen, regardless
of XEN_PV/PVH/PVHVM also makes sense from Linux POV.

That said, I don't see much value in removing XEN_PVHVM: why are we even
doing this? What is the improvement we are seeking?

> >> 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.
> And with a xen_guest_init() this should still need to be resolved, and
> I think I have this mostly addressed already in my dev branch, just
> have to clean it up.
>  Luis

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