Re: [Xen-devel] Radical proposal: ship not-fully-tidied shim as 4.10.1

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 9 Jan 2018, Doug Goldstein wrote:
> On 1/9/18 11:33 AM, Jan Beulich wrote:
> >>>> On 09.01.18 at 18:23, <anthony@xxxxxxxxxxxxx> wrote:
> >> On Tue, Jan 9, 2018 at 8:52 AM, Stefano Stabellini
> >> <sstabellini@xxxxxxxxxx> wrote:
> >>> On Tue, 9 Jan 2018, George Dunlap wrote:
> >>>> On Mon, Jan 8, 2018 at 9:01 PM, Rich Persaud <persaur@xxxxxxxxx> wrote:
> >>>>> On a similarly pragmatic note: would a variation of Anthony's vixen 
> >>>>> patch 
> >> series be suitable for pre-PVH Xen 4.6 - 4.9?  These versions are 
> >> currently 
> >> documented as security-supported (Oct 2018 - July 2020).
> >>>>
> >>>> Hmm, Ian's mail seems to be focusing on the idea of checking in a
> >>>> non-polished series to 4.10, rather than exctly what the content of
> >>>> that series would be.
> >>>>
> >>>> In the IRL conversation that preceeded this mail, the new short-term
> >>>> target we discussed was:
> >>>> 1. A 4.10-based shim that could boot either under HVM or PVH
> >>>> 2. A script that would take an existing PV config, and spit out a) a
> >>>> bootable ISO with the shim & whatever was needed, and b) a new config
> >>>> that would boot the same VM, but in HVM mode with the shim
> >>>>
> >>>> The script + a 4.10 shim binary *should* allow most PV guests to boot
> >>>> without any changes whatsoever for most older versions of Xen.
> >>>>
> >>>> There are a number of people for whom this won't work; I think we also
> >>>> need to provide a way to transparently change PV guests into PVshim
> >>>> guests.  But that will necessarily involve significant toolstack
> >>>> functionality, at which point you might as well backport PVH as well.
> >>>
> >>> Yes, there will be a number of people that won't be covered by this fix,
> >>> including those that can't use HVM/PVH mode because VT-x isn't available
> >>> at all in their environment. That is the only reason to run PV today.
> >>> Providing a way to transparently change PV guests into PVshim guests
> >>> won't cover any of these cases. A more complete workaround to SP3 is
> >>> along the lines of https://marc.info/?l=xen-devel&m=151509740625690.
> >>>
> >>> That said, I realize that we are only trying to do the best we can in a
> >>> very difficult situation, with very little time in our hands. I agree
> >>> with Ian that we should commit something unpolished and only partially
> >>> reviewed soon, even though it doesn't cover a good chunk of the userbase
> >>> for one reason or another. Even if migration doesn't work, it will still
> >>> help all that don't require it. It is only a partial fix by nature
> >>> anyway.
> >>
> >> Can people be a bit more explicit about what they think should be done 
> >> here?
> >>
> >> I'm happy to redirect effort to PVH shim if that's what the solution
> >> is going to be.
> >>
> >> I obviously prefer the HVM approach as it works on a broad range of Xen 
> >> versions
> >> without modification but I'm keen to get something done quickly and
> >> don't want to
> >> waste effort.
> > 
> > From what I've read today, I have no reason to believe the PVH
> > shim won't work in HVM mode. How would the HVM-only approach
> > be better in that case?
> > 
> > Jan
> 
> I feel like I should state the obvious here. Its tested over a large
> data set.

Right: if we are going to commit something unpolished and unreviewed,
let it be at least very well tested by the submitter. Honest question:
how much more dev&test we need on PVShim before we get it to similar
levels of confidence?

This is about an emergency stop-gap, we can work on a nice and shiny new
fix for the next release. We can even revert Anthony's series entirely
and start from scratch again, if that is required.

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