From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50099) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XovWb-0000JP-Nn for qemu-devel@nongnu.org; Thu, 13 Nov 2014 09:37:41 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XovWT-0003XY-7z for qemu-devel@nongnu.org; Thu, 13 Nov 2014 09:37:29 -0500 Received: from e37.co.us.ibm.com ([32.97.110.158]:59683) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XovWS-0003W8-UD for qemu-devel@nongnu.org; Thu, 13 Nov 2014 09:37:21 -0500 Received: from /spool/local by e37.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 13 Nov 2014 07:37:18 -0700 Message-ID: <5464C207.3020903@linux.vnet.ibm.com> Date: Thu, 13 Nov 2014 20:06:55 +0530 From: Aravinda Prasad MIME-Version: 1.0 References: <20141105071019.26196.93729.stgit@aravindap> <20141105071315.26196.68104.stgit@aravindap> <20141111031635.GF15270@voom.redhat.com> <5461B04F.5080204@linux.vnet.ibm.com> <20141113035206.GH7291@voom.fritz.box> <54644886.9050803@linux.vnet.ibm.com> <20141113103235.GK7291@voom.fritz.box> <54649A80.5000204@linux.vnet.ibm.com> <20141113124454.GB18600@voom.fritz.box> In-Reply-To: <20141113124454.GB18600@voom.fritz.box> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v3 4/4] target-ppc: Handle ibm, nmi-register RTAS call List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: David Gibson Cc: qemu-ppc@nongnu.org, benh@au1.ibm.com, aik@au1.ibm.com, qemu-devel@nongnu.org, paulus@samba.org 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