Re: [Xen-devel] [PATCH 00/22] Vixen: A PV-in-HVM shim

[George Dunlap <dunlapg@xxxxxxxxx>]

On Mon, Jan 8, 2018 at 4:02 PM, Anthony Liguori <anthony@xxxxxxxxxxxxx> wrote:
>>> I do want to make the shim be able to run in both pvh and hvm mode
>>> (which doesn't seem to be too hard in practice).
>>
>> AFAIK the pv-shim code will already work in HVM mode. It's just that
>> booting the pv-shim in HVM mode requires that you install the shim
>> inside of the guest and then boot it using grub or a similar loader
>> that can do multiboot.
>
> I'm happy to work on either approach.  I just want to get something
> merged to have
> an upstream solution to this issue.  I think this particular CVE for
> Xen PV is the worst
> of this batch of issues so I'm super eager on getting a solution
> straightened out.  I'd
> really like to hear from others on what the right approach should be
> and I'll work on
> whatever the consensus is.
>
> I think PVH is a good long term solution but I think it's a poor short
> term solution.
> PVH isn't widely deployed so it's asking people to upgrade their
> infrastructure to a
> very new version of Xen.  It also requires tools changes which means
> that even if
> you are on a newer version of Xen, you still have to upgrade.  The
> patch series is
> also pretty big which means I suspect people will need to wait to 4.11 at 
> best.
>
> OTOH, the HVM version of the series requires no tools changes and works on Xen
> versions going back to 3.4 (at least).  What this means practically
> speaking is that
> if it were merged, we can tell people that they can solve this problem
> by building the
> HVM shim and modifying their launch config to boot from an ISO or
> something similar.
>
> This gives people an immediate solution that does not require major
> changes to their
> underlying infrastructure.

Solving the "how to we boot the shim" question is the main reason that
we decided to start with PVH-only back to 4.8.

We didn't consider working around it by having a special boot disk
(ISO or otherwise); it's hard to know how well that will work for most
people.  You don't think that "having to create and boot from a custom
ISO" would count as "major changes to underlying infrastructure"?

> The series now is also reasonably contained and small enough that
> IMHO, it could go
> into the stable tree.  That means that once merged, we could cut a
> stable release giving
> people an official release that could be used for this purpose.
>
> If it was entirely my call, I would work on merging HVM shim first,
> get a 4.10 stable release
> cut with it, and then focus on getting PVH shim in place for the 4.11
> release.  I think
> this is the right balance of addressing the short term needs while
> also having the best long
> term solution.

If I understand correctly, this series is missing a number of features
from the other series -- migration being the key one, but perhaps
others (vcpu hot-plug? ballooning?).

In either case, it sounds like "additional boot disk" should work for
older versions, it sounds like.

 -George

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