Re: [PATCH v5 3/3] ppc: Enable 2nd DAWR support on p10

[Shivaprasad G Bhat <sbhat@linux.ibm.com>]

Hi David, All,

I am revisiting/reviving this patch.

On 5/5/21 11:20, David Gibson wrote:
> On Wed, Apr 21, 2021 at 11:50:40AM +0530, Ravi Bangoria wrote:
>> Hi David,
>>
>> On 4/19/21 10:23 AM, David Gibson wrote:
>>> On Mon, Apr 12, 2021 at 05:14:33PM +0530, Ravi Bangoria wrote:
>>>
<snip>
> Since we have released versions with POWER10 support, but no DAWR1, in
> theory we need a capability so new qemu with old machine types don't
> gain guest visible features that the same machine types on older qemus
> had.
>
> Except.. there's a loophole we might use to sidestep that.  The
> current POWER10 CPU modelled in qemu is a DD1 - which I strongly
> suspect will never appear outside of IBM.  I'm pretty sure we want to
> replace that with a DD2.
>
> While the modelled CPU is DD1, I think it's pretty reasonable to say
> our POWER10 support hasn't yet stabilized, and it would therefore be
> ok to simply add DAWR1 on POWER10 unconditionally, as long as we do it
> before we switch over to DD2.

As POWER10 DD2 switch over has already happened, the need for

new/separate capability for dawr1 still holds. So, I am keeping it as is.


Posting the next version after rebase.


Thanks,

Shivaprasad

>>> I'm wondering if we're actually just better off setting the pa feature
>>> just based on the guest CPU model.  TCG will be broken if you try to
>>> use it, but then, it already is.  AFAIK there's no inherent reason we
>>> couldn't implement DAWR support in TCG, it's just never been worth the
>>> trouble.
>> Correct. Probably there is no practical usecase for DAWR in TCG mode.
>>
>> Thanks,
>> Ravi
>>