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

[Rich Persaud <persaur@xxxxxxxxx>]

On Jan 11, 2018, at 11:36, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
> 
>> 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.

Would a “proper” PV fix (does this have a codename?) benefit stubdoms?  These 
are needed to isolate Qemu, e.g. on an HVM driver domain.  PVshim does not yet 
support driver domains.

Rich
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