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Date: Thu, 4 Mar 2021 06:36:20 +0530
From: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [PATCH v2 1/3] powerpc: sstep: Fix load and update emulation
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On 2021/03/04 09:45AM, Segher Boessenkool wrote:
> 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?

I tested this on two processors:
2.0 (pvr 004d 0200)
2.1 (pvr 004b 0201)

I guess the behavior changed some time during P8, but I don't have a P9 
to test this on.

In any case, this souldn't matter too much for us as you rightly point 
out:

> 
> 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.

Ack.


Thanks!
- Naveen