On Wed, Mar 03, 2021 at 10:01:27PM +0530, Naveen N. Rao wrote: > On 2021/03/01 08:37PM, Segher Boessenkool wrote: > > > 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.
Yes, I misread your code it seems. > 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. Yup. So on what cpu did you test? Either way, the kernel should not emulate any particular cpu here, I'd say, esp. since recent cpus do different things for this invalid form. Segher
