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Re: [Xen-devel] RFC: xen config changes v4

On Fri, 27 Feb 2015, Ian Campbell wrote:
> 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 PAE and x86_64 guests.

> 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.

Yes, I think that would be valuable.

> 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.

Right, this is what I am trying to avoid.  I would also like to avoid
having two levels of PVHVM guests for PAE enabled and x86_64 guests.

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