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

[Ravi Bangoria <ravi.bangoria@linux.ibm.com>]

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:
>> As per the PAPR, bit 0 of byte 64 in pa-features property indicates
>> availability of 2nd DAWR registers. i.e. If this bit is set, 2nd
>> DAWR is present, otherwise not. Use KVM_CAP_PPC_DAWR1 capability to
>> find whether kvm supports 2nd DAWR or not. If it's supported, allow
>> user to set the pa-feature bit in guest DT using cap-dawr1 machine
>> capability. Though, watchpoint on powerpc TCG guest is not supported
>> and thus 2nd DAWR is not enabled for TCG mode.
>>
>> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
>> Reviewed-by: Greg Kurz <groug@kaod.org>
> 
> So, I'm actually not sure if using an spapr capability is what we want
> to do here.  The problem is that presumably the idea is to at some
> point make the DAWR1 capability default to on (on POWER10, at least).
> But at that point you'll no longer to be able to start TCG guests
> without explicitly disabling it.  That's technically correct, since we
> don't implement DAWR1 in TCG, but then we also don't implement DAWR0
> and we let that slide... which I think is probably going to cause less
> irritation on balance.

Ok. Probably something like this is what you want?

Power10 behavior:
   - KVM does not support DAWR1: Boot the guest without DAWR1
     support (No warnings). Error out only if user tries with
     cap-dawr1=on.
   - KVM supports DAWR1: Boot the guest with DAWR1 support, unless
     user specifies cap-dawr1=off.
   - TCG guest: Ignore cap-dawr1 i.e. boot as if there is only
     DAWR0 (Should be fixed in future while adding PowerPC watch-
     point support in TCG mode)

Power10 predecessor behavior:
   - KVM guest: Boot the guest without DAWR1 support. Error out
     if user tries with cap-dawr1=on.
   - TCG guest: Ignore cap-dawr1 i.e. boot as if there is only
     DAWR0 (Should be fixed in future while adding PowerPC watch-
     point support in TCG mode)

> 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