Re: [Qemu-devel] [PATCH v3 4/4] target-ppc: Handle ibm, nmi-register RTAS call

[David Gibson]

On Thu, Nov 13, 2014 at 05:18:16PM +0530, Aravinda Prasad wrote:
> On Thursday 13 November 2014 04:02 PM, David Gibson wrote:
> > On Thu, Nov 13, 2014 at 11:28:30AM +0530, Aravinda Prasad wrote:
[snip]
> >>>>> Having to retry the hcall from here seems very awkward.  This is a
> >>>>> private hcall, so you can define it to do whatever retries are
> >>>>> necessary internally (and I don't think your current implementation
> >>>>> can fail anyway).
> >>>>
> >>>> Retrying is required in the cases when multi-processors experience
> >>>> machine check at or about the same time. As per PAPR, subsequent
> >>>> processors should serialize and wait for the first processor to issue
> >>>> the ibm,nmi-interlock call. The second processor retries if the first
> >>>> processor which received a machine check is still reading the error log
> >>>> and is yet to issue ibm,nmi-interlock call.
> >>>
> >>> Hmm.. ok.  But I don't see any mechanism in the patches by which
> >>> H_REPORT_MC_ERR will report failure if another CPU has an MC in
> >>> progress.
> >>
> >> h_report_mc_err returns 0 if another VCPU is processing machine check
> >> and in that case we retry. h_report_mc_err returns error log address if
> >> no other VCPU is processing machine check.
> > 
> > Uh.. how?  I'm only seeing one return statement in the implementation
> > in 3/4.
> 
> This part is in 4/4 which handles ibm,nmi-interlock call in
> h_report_mc_err()
> 
> +    if (mc_in_progress == 1) {
> +        return 0;
> +    }

Ah, right, missed the change to h_report_mc_err() in the later patch.

> >>>> 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.
> >>>
> >>> It's possible, but would require some fiddling inside the h_call to
> >>> unlock and wait for the other CPUs to finish, so yes, it might be more
> >>> trouble than it's worth.
> >>>
> >>>>
> >>>>>
> >>>>>> +      mtsprg  2,4
> >>>>>
> >>>>> Um.. doesn't this clobber the value of r3 you saved in SPRG2 just above.
> >>>>
> >>>> The r3 saved in SPRG2 is moved to rtas area in the private hcall and
> >>>> hence it is fine to clobber r3 here
> >>>
> >>> Ok, if you're going to do some magic register saving inside the HCALL,
> >>> why not do the SRR[01] and CR restoration inside there as well.
> >>
> >> SRR0/1 is clobbered while returning from HCALL and hence cannot be
> >> restored in HCALL. For CR, we need to do the restoration here as we
> >> clobber CR after returning from HCALL (the instruction checking the
> >> return value of hcall clobbers CR).
> > 
> > Hrm.  AFAICT SRR0/1 shouldn't be clobbered when returning from an
> 
> As hcall is an interrupt, SRR0 is set to nip and SRR1 to msr just before
> executing rfid.

AFAICT the return path from the hypervisor - including for hcalls -
uses HSSR0/1 and hrfid, so ordinary SRR0/SRR1 should be ok.

-- 
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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