Re: [Qemu-ppc] [RFC PATCH] target/ppc: extend eieio for POWER9

[David Gibson]

On Tue, Jun 05, 2018 at 10:14:13AM +0200, Cédric Le Goater wrote:
> On 06/05/2018 01:10 AM, David Gibson wrote:
> > On Mon, Jun 04, 2018 at 07:20:39PM +0200, Cédric Le Goater wrote:
> >> POWER9 introduced a new variant of the eieio instruction using bit 6
> >> as a hint to tell the CPU it is a store-forwarding barrier.
> >>
> >> The usage of this eieio extension was recently added in Linux 4.17
> >> which activated the "support for a store forwarding barrier at kernel
> >> entry/exit".
> >>
> >> This loosen the QEMU eieio instruction mask to boot newer kernel but I
> >> think we should be adding a new *eieio* instruction specific to POWER9
> >> instead. I just don't know how to define an instruction variant with
> >> the same op code for an ISA version. Any idea ?
> > 
> > I think you're right that this should be done slightly differently.
> > I think you can do that by adding a new instruction mask bit; say
> > PPC2_MEM_EIEIO2 or whatever.  You leave the existing GEN_HANDLER as
> > is, add another GEN_HANDLER_E with the new mask dependent on the new
> > bit, then make sure POWER9 has the new bit set, but not the old one.
> 
> Unfortunately this doesn't work :/ QEMU considers the opcode is already
> defined. May be we could test bit6 in gen_eieio ? 

Yeah, I guess we'll have to.

-- 
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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