[PATCH v2 1/3] powerpc: sstep: Fix load and update emulation

[Naveen N. Rao]

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