Re: [PATCH v3 1/8] iommu: Add ops entry for supported default domain type

[Lu Baolu]

Hi Robin,

On 5/10/19 12:11 AM, Robin Murphy wrote:
> On 09/05/2019 03:30, Lu Baolu wrote:
> > Hi Robin,
> >
> > On 5/7/19 6:28 PM, Robin Murphy wrote:
> > > On 06/05/2019 16:32, Tom Murphy via iommu wrote:
> > > > The AMD driver already solves this problem and uses the generic
> > > > iommu_request_dm_for_dev function. It seems like both drivers have the
> > > > same problem and could use the same solution. Is there any reason we
> > > > can't have use the same solution for the intel and amd driver?
> > > >
> > > > Could we just copy the implementation of the AMD driver? It would be
> > > > nice to have the same behavior across both drivers especially as we
> > > > move to make both drivers use more generic code.
> > >
> > > TBH I don't think the API really needs to be involved at all here. 
> > > Drivers can already not provide the requested default domain type if 
> > > they don't support it, so as long as the driver can ensure that the 
> > > device ends up with IOMMU or direct DMA ops as appropriate, I don't 
> > > see any great problem with drivers just returning a passthrough 
> > > domain when a DMA domain was requested, or vice versa (and logging a 
> > > message that the requested type was overridden). The only type that 
> > > we really do have to honour strictly is non-default (i.e. unmanaged) 
> > > domains.
> >
> > I agree with you that we only have to honor strictly the non-default
> > domains. But domain type saved in iommu_domain is consumed in iommu.c
> > and exposed to user through sysfs. It's not clean if the iommu driver
> > silently replace the default domain.
>
> Right, I did get a bit ahead of myself there - the implicit step before 
> that is to fix default domain allocation so that the core actually 
> passes the relevant device which it has to hand, such that the IOMMU 
> drivers *can* make the right decision up-front.

Yes, passing the relevant device when allocating the default domain so
that the IOMMU driver could make right decision seems to be a better
solution. Somebody can come up with a patch set to bring this up for
discussion. I won't include this in this patch set since it's not for
that purpose. I will follow the existing mechanism that is using on amd
and other iommu drivers.

Best regards,
Lu Baolu