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

[Aravinda Prasad <aravinda@linux.vnet.ibm.com>]

On Thursday 13 November 2014 06:14 PM, David Gibson wrote:
> 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.

I see SRR0 and SRR1 clobbered when the HCALL from guest returns.
Previous discussions on this is in the link below:

http://lists.nongnu.org/archive/html/qemu-devel/2014-09/msg01148.html

Further I searched QEMU source code but could not find whether it is
using rfid/hrfid. However, ISA for sc instruction mentions that SRR0 and
SRR1 are modified.


-- 
Regards,
Aravinda