Re: [PATCH] target/ppc: Implement ISA v3.1 wait variants

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, May 17, 2021 at 05:19:06PM +1000, Nicholas Piggin wrote:
> Excerpts from David Gibson's message of May 17, 2021 3:39 pm:
> > On Mon, May 17, 2021 at 12:46:51PM +1000, Nicholas Piggin wrote:
> >> ISA v3.1 adds new variations of wait, specified by the WC field. These
> >> are not compatible with the wait 0 implementation, because they add
> >> additional conditions that cause the processor to resume, which can
> >> cause software to hang or run very slowly.
> >> 
> >> Add the new wait variants with a trivial no-op implementation, which is
> >> allowed, as explained in comments: software must not depend on any
> >> particular architected WC condition having caused resumption of
> >> execution, therefore a no-op implementation is architecturally correct.
> >> 
> >> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > 
> > Logic looks fine.  There is no test on the CPU's features or model
> > here, though, so this will change behaviour for pre-3.1 CPUs as well.
> 
> Huh. 2.06-2.07 has very similar WC bits as 3.1, but 3.0 removed them
> and made them reserved. I should have looked back but I'd assumed
> they weren't there either.
> 
> Existing code treats WC != 0 as invalid on pre-3.0 processors AFAIKS,
> so that's not quite right for 2.06-7 (they should look more like 3.1).
> 
> But before that it looks like it was just wait with no WC field.
> 
> > What would invoking these wait variants (presumably reserved) on
> > earlier CPUs do?
> 
> Prior to 2.06, it looks like there is no WC field, and so they should 
> generate a program check. So that just leaves the incorrect program
> checks for 2.06-7, something like this should do it:
> 
> -GEN_HANDLER_E(wait, 0x1F, 0x1E, 0x00, 0x039FF801, PPC_NONE, PPC2_ISA300),
> +GEN_HANDLER_E(wait, 0x1F, 0x1E, 0x00, 0x039FF801, PPC_NONE, PPC2_ISA206),

Ok, can you update with such a change, and put some of this
explanation of the history in a comment.

> 2.06-3.1 should all be fine with this patch, AFAIKS they all have words 
> to the effect that WC != 0 is subject to implementation defined 
> behaviour and may be treated as a no-op or not implemented.

Ok.  Note that we do try to match specific CPU behaviour, not just the
architecture, although the architecture is obviously more important.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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