Re: RFC: xen config changes v4

[Ian Campbell <ian.campbell@citrix.com>]

On Fri, 2015-02-27 at 10:11 +0000, Stefano Stabellini wrote:

(for some reason I initially thought this was in reply to my mail, so
it's written in a way which assumes that, so sprinkle IMHO around the
place and/or take it as a follow on to my previous mail in this thread,
I guess)

> This is not a convincing explanation.  Debian, Ubuntu and Fedora seems
> to be able to cope with it just fine.

Debian doesn't really, for an i386 Debian installation you need to go
and find some slightly obscure media which has a PAE kernel on it in
order to install with PV drivers. If you just download the most obvious
i386 installation media you get no PV drivers of any description in an
HVM guest.

Fedora IIRC has moved everything over to PAE by default (no non-PAE
support), so they are probably OK.

I've no idea what Ubuntu does.

> Why do you want to do that, even though it will cause a performance
> regression and a maintenance pain?  You haven't provided a reason yet.

Where is the performance regression?

For a non-PAE x86 guest, which currently has 0 PV optimisations enabled
(no PV I/O, no PV clock, nothing) being able to enable PV I/O is a
useful performance improvement.

I'm also not saying that it *only* makes sense to enable PV I/O, if it
was also possible to enable other PV things, like PV clocks etc for
non-PAE x86 guests then that would also be worthwhile.

But I am saying that if enabling those extra optimisations for non-PAE
x86 guests is too invasive or problematic or whatever then it would
*still* be worth enabling PV I/O if that is more possible.

Note that in no case am I suggesting turning off something which is
possible today. In particular I see no reason to want to disable PV
optimisations for PAE enabled x86 guests.