Re: [Qemu-devel] [Qemu-ppc] [PATCH v3 0/4] target-ppc: Add FWNMI support in qemu for powerKVM guests

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Sep 04, 2015 at 12:00:25AM +0530, Aravinda Prasad wrote:
> 
> 
> On Thursday 03 September 2015 11:52 AM, Sam Bobroff wrote:
> > On Thu, Sep 03, 2015 at 03:05:21PM +1000, David Gibson wrote:
> > 
> > [snip]
> > 
> >> Hm.. so why can't the hypervisor code do the retrying?
> > 
> > Aravinda replied to this earlier in the thread:
> > 
> > "Retrying cannot be done internally in h_report_mc_err hcall: only one
> > thread can succeed entering qemu upon parallel hcall and hence retrying
> > inside the hcall will not allow the ibm,nmi-interlock from first CPU to
> > succeed."
> > 
> > I assume that this means that the big QEMU lock is held while an hcall is
> > processed by QEMU, but I haven't checked the code myself. Actually, even if the
> > lock is normally held, I don't see why these particular hcalls couldn't release
> > the lock. I'll look into this.
> 
> I am not sure whether we can release this lock inside an hcall. I need
> to check.

I don't see any reason that won't work.  As long as you only touch
most qemu data structures while the lock is held, of course.

> 
> > 
> >>>> Also, it looks like the vector will need at least one scratch register
> >>>> (for the hcall number, if nothing else).  Does PAPR specify what SPRGs
> >>>> the vector can clobber?  Obviously it can't be anything the guest
> >>>> kernel uses.
> >>>
> >>> PAPR only says SPRGs 0 to 3 are for software use, but the kernel (see
> >>> arch/powerpc/include/asm/reg.h) defines SPRG2 as an exception scratch register
> >>> so it should be the right one to use here.
> >>
> >> Uh.. no.  If 0..3 are for software (i.e. OS) use, then this needs to
> >> use a different one, since it's being used as a firmware resource
> >> here.  Linux might treat SPRG2 as scratch, but another OS would be
> >> within its rights to use it for something persistent.
> >>
> >> Although, as paulus points out, sc 1 will clobber SRR0/1 anyway, and
> >> if we use a special illegal instruction, then you no longer need a
> >> scratch register.
> >>
> >>>> Btw, does anyone know what happens with the VPA (and dispatch trace
> >>>> log and so forth) on kexec() - it could be subject to the same stale
> >>>> address problem, and rewriting vectors won't save us there.
> >>>
> >>> I asked Michael Ellerman this one and he thinks kexec probably frees and
> >>> re-allocates the VPA.
> >>
> >> Ok.  So the question is: if an explicit deregister is good enough for
> >> the VPA, is it also good enough for the FWNMI vector, in which case
> >> doing it with just a qemu exit and not bouncing through the guest space
> >> is back on the table.
> >>
> >> I guess that's still problematic because there are existing guests
> >> that assume a kexec() will magically wipe the fwnmi vectors away.
> > 
> > Yes, but I think we could handle this separately if necessary: even if we don't
> > need to write anything to the vector, we could still insert a magic value and
> > check for it later. If it's been clobbered by a kexec, go back to the old
> > method.
> 
> "> check for it later" - But does QEMU is informed or get to know when
> kexec() is issued?

No, but I think Sam is suggesting just rechecking the value when you
catch an MC exception.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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