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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

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.

 -George


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