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From: Daniel Axtens <dja@axtens.net>
To: Ravi Bangoria <ravi.bangoria@linux.ibm.com>, mpe@ellerman.id.au
Subject: Re: [PATCH v5 1/5] powerpc/sstep: Emulate prefixed instructions only
 when CPU_FTR_ARCH_31 is set
In-Reply-To: <20201011050908.72173-2-ravi.bangoria@linux.ibm.com>
References: <20201011050908.72173-1-ravi.bangoria@linux.ibm.com>
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Date: Tue, 13 Oct 2020 00:44:14 +1100
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Hi,

To review this, I looked through the supported instructions to see if
there were any that I thought might have been missed.

I didn't find any other v3.1 ones, although I don't have a v3.1 ISA to
hand so I was basically looking for instructions I didn't recognise.

I did, however, find a number of instructions that are new in ISA 3.0
that aren't guarded:

 - addpcis
 - lxvl/stxvl
 - lxvll/stxvll
 - lxvwsx
 - stxvx
 - lxsipzx
 - lxvh8x
 - lxsihzx
 - lxvb16x/stxvb16x
 - stxsibx
 - stxsihx
 - lxvb16x
 - lxsd/stxsd
 - lxssp/stxssp
 - lxv/stxv
 
Also, I don't know how bothered we are about P7, but if I'm reading the
ISA correctly, lqarx/stqcx. are not supported before ISA 2.07. Likewise
a number of the vector instructions like lxsiwzx and lxsiwax (and the
companion stores).

I realise it's not really the point of this particular patch, so I don't
think this should block acceptance. What I would like to know - and
maybe this is something where we need mpe to weigh in - is whether we
need consistent guards for 2.07 and 3.0. We have some 3.0 guards already
but clearly not everywhere.

With all that said - the patch does what it says it does, and looks good
to me:

Reviewed-by: Daniel Axtens <dja@axtens.net>


Kind regards,
Daniel

> From: Balamuruhan S <bala24@linux.ibm.com>
>
> Unconditional emulation of prefixed instructions will allow
> emulation of them on Power10 predecessors which might cause
> issues. Restrict that.
>
> Fixes: 3920742b92f5 ("powerpc sstep: Add support for prefixed fixed-point arithmetic")
> Fixes: 50b80a12e4cc ("powerpc sstep: Add support for prefixed load/stores")
> Signed-off-by: Balamuruhan S <bala24@linux.ibm.com>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
> ---
>  arch/powerpc/lib/sstep.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
> index e9dcaba9a4f8..e6242744c71b 100644
> --- a/arch/powerpc/lib/sstep.c
> +++ b/arch/powerpc/lib/sstep.c
> @@ -1346,6 +1346,9 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
>  	switch (opcode) {
>  #ifdef __powerpc64__
>  	case 1:
> +		if (!cpu_has_feature(CPU_FTR_ARCH_31))
> +			return -1;
> +
>  		prefix_r = GET_PREFIX_R(word);
>  		ra = GET_PREFIX_RA(suffix);
>  		rd = (suffix >> 21) & 0x1f;
> @@ -2733,6 +2736,9 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
>  		}
>  		break;
>  	case 1: /* Prefixed instructions */
> +		if (!cpu_has_feature(CPU_FTR_ARCH_31))
> +			return -1;
> +
>  		prefix_r = GET_PREFIX_R(word);
>  		ra = GET_PREFIX_RA(suffix);
>  		op->update_reg = ra;
> -- 
> 2.26.2