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Re: Utility of PV on non-obsolete hardware

On Sat, Jun 01, 2019 at 05:42:12PM +0200, Maxime Villard wrote:
> I hope we will be able to retire XenPV sooner than later. We have too many
> shades of x86 (between native-32, native-32-pae, native-64, XenPV-32-pae,
> XenPV-64, XenPVHVM*), and it is becoming impossible to understand what the
> code really does. Meanwhile, the x86 architecture is also getting increasingly
> complex, and there are planned future changes that will modify the paging
> logic. If we ever want to support any of that, we will either have to retire
> certain configurations, or split the code and give a separate pmap (among
> other things) to the different configurations.
> This would ease maintenance, which would ease support for future features
> and also the already existing ones.
> For example the other day I was playing with tprof, and noticed that about 10%
> of the TLB misses we get during a build.sh are caused by the recursive PTE
> slot. If we were using the direct map, almost all of these misses would vanish.
> Should I add another ton of #ifdefs in all directions just to get native-64 to
> use the direct map in order to reduce TLB misses?

But this is not because of PV (actually using recursive PTEs make things
harder for PV too), but because the pmap is shared with i386.

Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference

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