Re: Xen MP panics in cpu_switchto()

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Mon, Jan 13, 2020 at 07:11:21PM +0000, Andrew Doran wrote:
> On Mon, Jan 13, 2020 at 07:36:41PM +0100, Manuel Bouyer wrote:
> 
> > On Mon, Jan 13, 2020 at 06:33:08PM +0000, Andrew Doran wrote:
> > > On Mon, Jan 13, 2020 at 05:43:51PM +0100, Manuel Bouyer wrote:
> > > 
> > > > On Mon, Jan 13, 2020 at 04:59:50PM +0100, Manuel Bouyer wrote:
> > > > > It also sets rsp and rbp. I think rbp is not set by anything else, at last
> > > > > in the Xen case.
> > > > > The different rbp value would explain why in one case we hit a KASSERT()
> > > > > in lwp_startup later.
> > > > > But I don't know what pcb_rbp contains; I couldn't find where the pcb for
> > > > > idlelwp is initialized.
> > > > 
> > > > I tried the attached patch, which should set rsp/rbp as cpu_switchto()
> > > > does. It doens't cause the lwp_startup() KASSERT as calling cpu_switchto()
> > > > does; it also doesn't change the scheduler behavior.
> > > 
> > > Wait - do you mean that everything works now?  Or that everything still runs
> > > on CPU0?
> > 
> > No, everything still runs on CPU0
> 
> Hmm, I don't understand why.  I'll set up Xen and try it out.  It might take
> me a day or two.

OK thanks. 

> [...]
> 
> The assertion in lwp_startup() is because I made MI changes so that prevlwp
> is never NULL when calling cpu_switchto(), when fixing some bugs problems MP
> support on !x86 and make things more correct.  lwp_startup()/mi_switch() now
> need to unlock prevlwp after it is finished in cpu_switchto().  I never
> expected anybody but mi_switch() to call cpu_switchto().

OK, so I removed the call to cpu_switchto() before idle_loop(),
and added a few KASSERTS.
I guess you can back out the prev == NULL case from cpu_switchto().

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