Re: [Xen-devel] [RFC] xen/pvh: detect PVH after kexec

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

On Tue, Mar 21, 2017 at 04:42:12AM -0600, Jan Beulich wrote:
> >>> On 21.03.17 at 11:21, <roger.pau@xxxxxxxxxx> wrote:
> > On Tue, Mar 21, 2017 at 04:07:51AM -0600, Jan Beulich wrote:
> >> >>> On 21.03.17 at 11:01, <roger.pau@xxxxxxxxxx> wrote:
> >> > On Tue, Mar 21, 2017 at 10:21:52AM +0100, Vitaly Kuznetsov wrote:
> >> >> Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> writes:
> >> >> 
> >> >> > On 03/20/2017 02:20 PM, Vitaly Kuznetsov wrote:
> >> >> >> PVH guests after kexec boot like normal HVM guests and we're not 
> >> >> >> entering
> >> >> >> xen_prepare_pvh()
> >> >> >
> >> >> > Is it not? Aren't we going via xen_hvm_shutdown() and then
> >> >> > SHUTDOWN_soft_reset which would restart at the same entry point as
> >> >> > regular boot?
> >> >> 
> >> >> No, we're not doing regular boot: from outside of the guest we don't
> >> >> really know where the new kernel is placed (as guest does it on its
> >> >> own). We do soft reset to clean things up and then guest jumps to the
> >> >> new kernel starting point by itself.
> >> >> 
> >> >> We could (in theory, didn't try) make it jump to the PVH starting point
> >> >> but we'll have to at least prepare the right boot params for
> >> >> init_pvh_bootparams and this looks like additional
> >> >> complication. PVHVM-style startup suits us well but we still need to be
> >> >> PVH-aware.
> >> > 
> >> > We are going to have the same issue when booting PVH with OVMF, Linux 
> >> > will 
> > be
> >> > started at the native UEFI entry point, and we will need some way to 
> >> > detect
> >> > that we are running in PVH mode.
> >> 
> >> I'm confused: PVH boots without any firmware, doesn't it? Hence
> >> it shouldn't matter if there's no (legacy) BIOS or no OVMF ...
> > 
> > Right now yes, we have no firmware available to PVH at all, but Anthony is
> > already working on porting OVMF to PVH [0].
> 
> But that leaves open the "why" aspect: What use is OVMF to a
> PVH guest?

IMHO it's better than pvgrub that Xen has been using for PV guests, and has the
bonus that OVMF can probably chainload grub, the FreeBSD loader or whatever
needed, giving us a lot of flexibility inside PVH guests.

To put a simple example, right now Xen cannot boot FreeBSD PVH guests with
modules, because the ramdisk option cannot be used by it (FreeBSD doesn't have
a ramdisk, the loader loads the needed modules at run-time). If PVH support is
added to OVMF, I should be able to chainload the FreeBSD EFI loader into it and
boot a FreeBSD guest with modules.

Roger.

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