Re: Runstate hypercall and Linux KPTI issues

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 10 Sep 2020, Jan Beulich wrote:
> >>> - should we backport the support for this hypercall in older kernel 
> >>> releases ?
> >>
> >> It's a bug fix to KPTI, and as such ought to be at least eligible for
> >> considering doing so?
> > 
> > That will mean also backport in Linux. What should be the scope ?
> 
> All longterm and stable trees which are affected, as I think is usual.

>From a Linux perspective, we should fix this as soon as possible: we
should backport a patch to make the usage of the runstate hypercall
conditional on KPTI being disabled on ARM.

Then, when available in Xen, we should backport the usage of the new
hypercall with a check to detect if it is available -- do not assume it
is available, users might not update Xen, but might update Linux.

We have to do this in two stages for a couple of reasons. It is best not
to wait for Xen-side changes to fix Linux any longer. And also, a Linux
fix is best implemented independently anyway: Linux should benefit from
the Xen improvements when available but not rely on their presence to
work.