Re: [Xen-devel] RFC: making the PVH 64bit ABI as stableo

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 06/02/2015 12:51 PM, Stefano Stabellini wrote:
> On Tue, 2 Jun 2015, Jan Beulich wrote:
> > > > > On 02.06.15 at 17:11, <roger.pau@xxxxxxxxxx> wrote:
> > > Hello,
> > >
> > > The document describing the PVH interface was committed 9 months ago
> > > [1], and since then there hasn't been any change regarding the
> > > interface. PVH is still missing features in order to have feature parity
> > > with pure PV, mainly:
> > >
> > >   - DomU miration support.
> > >   - PCI passthrough support.
> > >   - 32bit support.
> > >   - AMD support.
> > >
> > > AFAICT however none of these features are going to change the current
> > > ABI.
> > This your guess; I don't think there's any guarantee.
> Let's make it a guarantee.
>
>
> > The more that talk was about making PVH uniformly enter the kernel in
> > 32-bit mode.
> What talk? IRL talks are irrelevant in this context. If it is not on the
> list, it doesn't exist.
>
>
> > > PCI passthrough might expand it, by adding new hypercalls, but I
> > > don't think this should prevent us from marking the current ABI as
> > > stable. ARM for example doesn't have PCI passthrough yet or migration
> > > support, but the ABI has been marked as stable.
> > >
> > > To that end, I would like to request the 64bit PVH ABI to be marked as
> > > stable for DomUs. This is needed so external projects (like PVH support
> > > for grub2) can progress.
> > Understandable, but no, not before all the fixmes in the tree got
> > dealt with.
> What is your timeline for that? In fact does anybody have any timelines
> for it?
>
> We need to have a clear idea of what exactly needs to happen. We also
> need to have confidence that is going to happen in a reasonable time
> frame. At the moment we have some various mumblings about things, but we
> don't have a clear breakdown of the outstanding work and names
> associated with each work item.
>
> Is anybody going to volunteer writing that todo list?
>
> Are we going to be able to find enough volunteers with the right skills
> to be confident that PVH is going to be out of "experimental" within a
> reasonable time frame? It is clear that some of the clean-ups require an
> hypervisor expert.
>
> If not, I suggest we rethink our priorities and we consider dropping PVH
> entirely. I don't think is fair to expect Roger or anybody else to keep
> their efforts up on PVH, when actually we don't know if we'll be able to
> land it.


Roger, Tim, Elena, Konrad and I had a conversation a few months ago and 
at that time we came up with a (somewhat informal) list of what we knew 
was broken:
  - 32-bit cannot boot.
  - Does not work under AMD.
  - Migration
  - PCI passthrough
  - Memory ballooning
  - Multiple VBDs, NICs, etc.
  - CPUID filtering. There are no filtering done at all which means
that certain cpuid flags are exposed to the guest.
  - The x2apic will cause a crash if the NMI handler is invoked.
  - The APERF will cause inferior scheduling decisions.
  - working with shadow code (which is what we use when migrating HVM
guests). But the nice side-benefit is that we can then run PVH on
machines without VMX or SVM support.
  - TSC modes are broken.

Obviously some things in this list are large(er) projects and some are 
simply bugs.
I picked 32-bit support, Elena is looking into AMD and Roger agreed to 
look at migration and passthrough for now. Plus, at some point we will
probably need to think about how to move PVH support into a 
"feature-flag" support that Tim proposed at a hackathon last year (where 
we don't have HVM/PV/PVH guests but rather guests with various features 
enabled).
(Incidentally, I booted a 32-bit PVH guest yesterday finally. UP only 
for now)
So there is a more than one person working on it (for a specific 
definition of the word "working" since we all are constantly preempted 
by other things that also preemptable by more important things).
-boris


> Maybe we could focus on improving PV on HVM and its security. Maybe we
> could resurrect Intel's HVM Dom0 project
> (http://events.linuxfoundation.org/sites/events/files/slides/HVM%20Dom0.pdf).
> Think of how much farther we would be if we didn't start PVH in the
> first place.
>
>
> P.S.
> This message is not addressed to Jan in particular but to the larger Xen
> community.

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