Re: CVS commit: src/sys/arch/xen/xen

[David Laight <david%l8s.co.uk@localhost>]

On Sun, Feb 02, 2014 at 01:27:45PM +0100, Manuel Bouyer wrote:
> On Sat, Feb 01, 2014 at 10:49:44PM +0000, David Laight wrote:
> > On Sat, Feb 01, 2014 at 08:07:07PM +0000, Manuel Bouyer wrote:
> > > Module Name:      src
> > > Committed By:     bouyer
> > > Date:             Sat Feb  1 20:07:07 UTC 2014
> > > 
> > > Modified Files:
> > >   src/sys/arch/xen/xen: hypervisor.c
> > > 
> > > Log Message:
> > > Revert previous: calling fpuinit() leads to a panic, as a domU is not
> > > allowed to manipulate cr0 directly. Xen doesn't need this, the fpu is
> > > handled by the hypervisor.
> > 
> > I probably remember seeing that panic as well.
> > 
> > XEN might need the bits of fpuinit() that don't play with cr0.
> > In particular a fninit and setting TS.
> > Although I'm not 100% sure the TS is needed on a single cpu system.
> > It isn't there on amd64, but really it does no harm.
> > Without it one process will have an indererminate fp state.
> 
> It's been a while since I looked at this and I don't remmeber the details,
> but I don't think anything of fpuinit() is neeed for Xen. in netbsd-6
> npxattach() doens't do anything either for Xen (only set i386_fpu_present to
> 1 and init npxdna_func, which seems to be done at compile time now).
> I think the FPU is initialised by the hypervisor. Or it's done by
> npxdna() on the first FPU use.
> 
> With the code as is in HEAD now, all FPU-related atf tests pass on
> a Xen guest with 4 CPUs, so I assume it's working as expected.

Something needs to set the TS (task switched) flag when a new cpu
is added. Both amd64 and i386 'bare metal' have direct calls to
fpuinit() to do this.

If TS isn't set the first FP process that gets migrated to that cpu
won't fault on its first FP instruction. If it has just forked it
probably won't care, but otherwise it will all go horribly wrong.

It probably doesn't matter for the boot cpu - except that you (effectively)
clone 'random' FP registers instead of known 'initially zero' ones.

        David

-- 
David Laight: david%l8s.co.uk@localhost