RE: [PATCH 1/1] iommu/vt-d: Disable PCI ATS in legacy passthrough mode

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Baolu Lu <baolu.lu@linux.intel.com>
> Sent: Thursday, November 16, 2023 3:35 PM
> 
> On 2023/11/14 9:10, Lu Baolu wrote:
> > When IOMMU hardware operates in legacy mode, the TT field of the
> context
> > entry determines the translation type, with three supported types (Section
> > 9.3 Context Entry):
> >
> > - DMA translation without device TLB support
> > - DMA translation with device TLB support
> > - Passthrough mode with translated and translation requests blocked
> >
> > Device TLB support is absent when hardware is configured in passthrough
> > mode.
> >
> > Disable the PCI ATS feature when IOMMU is configured for passthrough
> > translation type in legacy (non-scalable) mode.
> >
> > Fixes: 0faa19a1515f ("iommu/vt-d: Decouple PASID & PRI enabling from
> SVA")
> > Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> > ---
> >   drivers/iommu/intel/iommu.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> > index 11670cd812a3..c3ec09118ab1 100644
> > --- a/drivers/iommu/intel/iommu.c
> > +++ b/drivers/iommu/intel/iommu.c
> > @@ -1413,6 +1413,10 @@ static void iommu_enable_pci_caps(struct
> device_domain_info *info)
> >   	if (!dev_is_pci(info->dev))
> >   		return;
> >
> > +	if (!sm_supported(info->iommu) && info->domain &&
> > +	    domain_type_is_si(info->domain))
> > +		return;
> > +
> >   	pdev = to_pci_dev(info->dev);
> >
> >   	/* The PCIe spec, in its wisdom, declares that the behaviour of
> 
> Perhaps we could move the check into the caller and make this helper
> transparent to the iommu mode and domain type?
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 11670cd812a3..9bddd4fbbdf8 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -2492,7 +2492,8 @@ static int dmar_domain_attach_device(struct
> dmar_domain *domain,
>                  return ret;
>          }
> 
> -       iommu_enable_pci_caps(info);
> +       if (sm_supported(info->iommu) || !domain_type_is_si(info->domain))
> +               iommu_enable_pci_caps(info);
> 

IMHO both old and this new version are confusing regarding to that
the commit msg talks only about ATS but the actual code disable all
pci caps. It's correct, being that only ATS is relevant in legacy mode,
but the readability is not good.

what about introducing a helper e.g. device_domain_ats_supported(info)
which includes above checks plus info->ats_supported and then use it
to replace info->ats_supported in iommu_enable_pci_caps()?