Re: [Xen-devel] Radical proposal v2: Publish Amazon's verison now, Citrix's version soon

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 11 Jan 2018, George Dunlap wrote:
> On 01/11/2018 04:23 PM, Stefano Stabellini wrote:
> > On Thu, 11 Jan 2018, Jan Beulich wrote:
> >>>>> On 10.01.18 at 18:25, <sstabellini@xxxxxxxxxx> wrote:
> >>> On Wed, 10 Jan 2018, George Dunlap wrote:
> >>>> * Executive summary
> >>>>
> >>>> - We've agreed on a "convergence" point for PV shim functionality that
> >>>>   covers as many users as possible:
> >>>>  - 'HVM' functionality: boots in HVM mode, has support for Xen 3.4
> >>>>    event channels, &c, booted via 'sidecar'
> >>>>  - 'PVH' functionality: boots in PVH mode, booted via toolstack
> >>>>    changes
> >>>>
> >>>> - "Vixen" (the Amazon shim) and PVH shim (mostly developed by Citrix)
> >>>>   each cover some users and not others; neither one (yet) covers all
> >>>>   users
> >>>
> >>> Sorry for being punctilious, but neither one can cover all users: there
> >>> are users without VT-x on their platform, and both approaches require
> >>> VT-x.
> >>
> >> For the record, yesterday I've decided to make an attempt to
> >> create a very simplistic patch to deal with the issue in the
> >> hypervisor, ignoring (almost) all performance considerations
> >> (not all, because I didn't want to go the "disable caching" route).
> >> I've dealt with some of the to-be-expected early bugs, but I'm
> >> now debugging a host hang (note: not a triple fault apparently,
> >> as the box doesn't reboot, yet triple faults is what I would have
> >> expected to occur if anything is wrong here or missing).
> >>
> >> I know that's late, and I have to admit that I don't understand
> >> myself why I didn't consider doing such earlier on, but the
> >> much increased pressure to get something like the shim out,
> >> which
> >> - doesn't address all cases
> >> - requires changes to how VMs are being created (which likely will
> >>   be a problem for various customers)
> >> - later will want those changes undone
> >> plus the pretty obvious impossibility to backport something like
> >> Andrew's (not yet complete) series to baselines as old as 3.2
> >> made it seem to me that some (measurable!) performance
> >> overhead can't be all that bad in the given situation.
> > 
> > Thank you for giving it a look! I completely agree with you on these
> > points. I think we should approach this problem with the assumption that
> > this is going to be the only long term solution to SP3, while Vixen (or
> > PVshim) incomplete stopgaps for now.
> 
> Well the pvshim is a feature for people who want to be able to eliminate
> all PV interfaces to the hypervisor whatsover for security / maintenance
> purposes.  I do agree a "proper" fix for PV would be good, assuming the
> overhead is lower than pvshim.

Why "assuming the overhead is lower than pvshim"? What if the overhead
is higher?  As I said, there are users that *cannot* deploy HVM because
it is not available to them.

In other words, PVshim is irrelevant to me because I cannot use it.

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