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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 24 Feb 2015, Luis R. Rodriguez wrote:
> On Tue, Feb 24, 2015 at 7:21 AM, Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > 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:
> >> >>> XEN_PVHVM
> >> >>
> >> >> 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.
> 
> OK great, thanks for the feedback.
> 
> > 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?
> 
> We would not, the above discussed about the possibility of letting
> users enable XEN_PVHVM without XEN_PVH, that's all.

OK, that makes sense.


> As is the only thing that can enable XEN_PVHVM is if you enable
> XEN_PVH.

This is the bit that we need to change but it shouldn't be difficult.


> If we want
> xen_guest_init() alone though we might need the decoupling though at
> least at build time so that if XEN_PV or XEN_PVH is not selected we'd
> at least have XEN_PVHVM. Thoughts?

Today pv(h) and pvhvm have very different boot paths already: pv and pvh
initialize via xen_start_kernel while pvhvm via xen_hvm_guest_init.
They are already very decoupled. You just need to make sure that
init_hvm_pv_info() is called regardless on any XEN_PV/PVH/PVHVM configs.

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