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Rao" To: Segher Boessenkool Subject: Re: [PATCH v2 1/3] powerpc: sstep: Fix load and update emulation Message-ID: <20210303163127.GE1913@DESKTOP-TDPLP67.localdomain> References: <20210203063841.431063-1-sandipan@linux.ibm.com> <20210203094909.GD210@DESKTOP-TDPLP67.localdomain> <20210203211732.GD30983@gate.crashing.org> <20210204082753.GI210@DESKTOP-TDPLP67.localdomain> <20210302023732.GH29191@gate.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210302023732.GH29191@gate.crashing.org> X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.369, 18.0.761 definitions=2021-03-04_03:2021-03-03, 2021-03-04 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 bulkscore=0 mlxlogscore=999 impostorscore=0 phishscore=0 spamscore=0 malwarescore=0 adultscore=0 suspectscore=0 priorityscore=1501 mlxscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2103040063 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: ravi.bangoria@linux.ibm.com, ananth@linux.ibm.com, jniethe5@gmail.com, paulus@samba.org, Sandipan Das , linuxppc-dev@lists.ozlabs.org, dja@axtens.net Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 2021/03/01 08:37PM, Segher Boessenkool wrote: > Hi! > > I didn't see this until now, almost a month later, sorry about that :-) No problem. > > On Thu, Feb 04, 2021 at 01:57:53PM +0530, Naveen N. Rao wrote: > > On 2021/02/03 03:17PM, Segher Boessenkool wrote: > > > Power8 does: > > > > > > Load with Update Instructions (RA = 0) > > > EA is placed into R0. > > > Load with Update Instructions (RA = RT) > > > EA is placed into RT. The storage operand addressed by EA is > > > accessed, but the data returned by the load is discarded. > > > > I'm actually not seeing that. This is what I am testing with: > > li 8,0xaaa > > mr 6,1 > > std 8,64(6) > > #ldu 6,64(6) > > .long 0xe8c60041 > > > > And, r6 always ends up with 0xaea. It changes with the value I put into > > r6 though. > > That is exactly the behaviour specified for p8. 0aaa+0040=0aea. > > > Granted, this is all up in the air, but it does look like there is more > > going on and the value isn't the EA or the value at the address. > > That *is* the EA. The EA is the address the insn does the access at. I'm probably missing something here. 0xaaa is the value I stored at an offset of 64 bytes from the stack pointer (r1 is copied into r6). In the ldu instruction above, the EA is 64(r6), which should translate to r1+64. The data returned by the load would be 0xaaa, which should be discarded per the description you provided above. So, I would expect to see a 0xc0.. address in r6. In fact, this looks to be the behavior documented for P9: > > Power9 does: > > > > Load with Update Instructions (RA = 0) > > EA is placed into R0. > > Load with Update Instructions (RA = RT) > > The storage operand addressed by EA is accessed. The > > displacement > > field is added to the data returned by the load and placed into > > RT. - Naveen