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Date: Mon, 1 Mar 2021 20:37:33 -0600
From: Segher Boessenkool <segher@kernel.crashing.org>
To: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 1/3] powerpc: sstep: Fix load and update emulation
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Hi!

I didn't see this until now, almost a month later, sorry about that :-)

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.


Segher