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

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

On 2021/02/03 03:17PM, Segher Boessenkool wrote:
> On Wed, Feb 03, 2021 at 03:19:09PM +0530, Naveen N. Rao wrote:
> > On 2021/02/03 12:08PM, Sandipan Das wrote:
> > > The Power ISA says that the fixed-point load and update
> > > instructions must neither use R0 for the base address (RA)
> > > nor have the destination (RT) and the base address (RA) as
> > > the same register. In these cases, the instruction is
> > > invalid.
> 
> > > However, the following behaviour is observed using some
> > > invalid opcodes where RA = RT.
> > > 
> > > An userspace program using an invalid instruction word like
> > > 0xe9ce0001, i.e. "ldu r14, 0(r14)", runs and exits without
> > > getting terminated abruptly. The instruction performs the
> > > load operation but does not write the effective address to
> > > the base address register. 
> > 
> > While the processor (p8 in my test) doesn't seem to be throwing an 
> > exception, I don't think it is necessarily loading the value. Qemu 
> > throws an exception though. It's probably best to term the behavior as 
> > being undefined.
> 
> 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.

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.

> 
> 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.
> 
> Both UMs also say
> 
>   Invalid Forms
>     In general, the POWER9 core handles invalid forms of instructions in
>     the manner that is most convenient for the particular case (within
>     the scope of meeting the boundedly-undefined definition described in
>     the Power ISA). This document specifies the behavior for these
>     cases.  However, it is not recommended that software or other system
>     facilities make use of the POWER9 behavior in these cases because
>     such behavior might be different in another processor that
>     implements the Power ISA.
> 
> (or POWER8 instead of POWER9 of course).  Always complaining about most
> invalid forms seems wise, certainly if not all recent CPUs behave the
> same :-)

Agreed.

- Naveen