Re: [RFC][PATCH] tracing/x86: Save CR2 before tracing irqsoff on error_entry

[Andy Lutomirski]

On Thu, Mar 21, 2019 at 11:37 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Mar 21, 2019 at 11:25:44AM -0700, Linus Torvalds wrote:
> > On Thu, Mar 21, 2019 at 11:21 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > >
> > > I dunno.  Lots of people at least use to have serious commercial interest in it.
> >
> > Yes, it used to be a big deal. But full virtualization has gotten a
> > lot more common and better.
> >
> > > Hey Xen folks, how close are we to being able to say "if you want to
> > > run a new kernel, you need to switch to PVH or similar"?
> >
> > I'd also like to know if we could perhaps at least limit PV to just
> > the thing that people care most deeply about.
> >
> > For example, maybe people notice that they really deeply care about
> > the PV spinlocks because they help a lot for some loads, but don't
> > care so much about the low-level CPU PV stuff any more because modern
> > CPUs do _those_ things so well these days.
> >
> > So it might not be an all-or-nothing thing, but a gradual "let's stop
> > supporting xyz under PV, because it causes pain and isn't worth it".
>
> So Juergen recently introduced PARAVIRT_XXL, which are exactly those
> bits of PV we can get rid of.
>
> This paravirt-me-harder config does indeed include the CR2 bits.
>
> I recently talked to Andrew Cooper about this, and he said Xen Dom0
> still needs all this :/

There were patches from last year to fix that:

https://lwn.net/Articles/753982/

I have no clue what the status is.