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

[Cédric Le Goater]

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 ? 
 

C.