Re: [PATCH RFC 04/10] intel_iommu: Second Stage Access Dirty bit support

[Joao Martins]

On 4/29/22 19:21, Peter Xu wrote:
> On Fri, Apr 29, 2022 at 10:12:01AM +0100, Joao Martins wrote:
>> On 4/29/22 03:26, Jason Wang wrote:
>>> On Fri, Apr 29, 2022 at 5:14 AM Joao Martins <joao.m.martins@oracle.com> 
>>> wrote:
>>>> @@ -3693,7 +3759,8 @@ static void vtd_init(IntelIOMMUState *s)
>>>>
>>>>      /* TODO: read cap/ecap from host to decide which cap to be exposed. */
>>>>      if (s->scalable_mode) {
>>>> -        s->ecap |= VTD_ECAP_SMTS | VTD_ECAP_SRS | VTD_ECAP_SLTS;
>>>> +        s->ecap |= VTD_ECAP_SMTS | VTD_ECAP_SRS | VTD_ECAP_SLTS |
>>>> +                   VTD_ECAP_SLADS;
>>>>      }
>>>
>>> We probably need a dedicated command line parameter and make it compat
>>> for pre 7.1 machines.
>>>
>>> Otherwise we may break migration.
>>
>> I can gate over an 'x-ssads' option (default disabled). Which reminds me 
>> that I probably
>> should rename to the most recent mnemonic (as SLADS no longer exists in 
>> manuals).
>>
>> If we all want by default enabled I can add a separate patch to do so.
> 
> The new option sounds good.
> 

OK, I'll fix it then for the next iteration.

Also, perhaps I might take the emulated iommu patches out of the iommufd stuff 
into a
separate series. There might be a place for them in the realm of 
testing/prototyping.

> Jason, per our previous discussion, shall we not worry about the
> compatibility issues per machine-type until the whole feature reaches a
> mostly-complete stage?
> 
> There seems to have a bunch of sub-features for scalable mode and it's a
> large project as a whole.  I'm worried trying to maintain compatibilities
> for all the small sub-features could be an unnessary burden to the code
> base.

Perhaps best to see how close we are to spec is to check what we support in 
intel-iommu
in terms of VT-d revision versus how many buckets we fill in. I think 
SLADS/SSADS was in
3.0 IIRC.

I can take the compat stuff out if it's too early for that -- But I take it
these are questions for Jason.