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

[Andy Lutomirski]

On Thu, Mar 21, 2019 at 11:27 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Mar 21, 2019 at 11:05:06AM -0700, Andy Lutomirski wrote:
>
> > Ugh.
> >
> > I certainly agree in principle that sticking the CR2 read into the asm
> > is the right solution.  But this patch makes the spaghetti even more
> > tangled.  Maybe we can rearrange the code a bit so that the entry
> > sequence saves at least one register before calling error_entry, so we
> > can do it the obvious way.
>
> What is the obvious way? Note that with all the Xen/paravirt-me-harder
> bits on, we need to clobber at least RAX to get CR2.
>
> And most of the patch was munging the paravirt bits to make that happen.
>
> Another thing I did notices is that CLAC and CLD are not next to one
> another in these paths.

The thing I really don't like is all the EBX crud to try to get
everything in to the right order.