Re: [PATCH v7 01/10] iommu: Add DOMAIN_ATTR_MSI_MAPPING attribute

[Robin Murphy]

On 20/04/16 16:58, Eric Auger wrote:
> Hi Robin,
> On 04/20/2016 02:47 PM, Robin Murphy wrote:
> > Hi Eric,
> >
> > On 19/04/16 17:56, Eric Auger wrote:
> > > Introduce a new DOMAIN_ATTR_MSI_MAPPING domain attribute. If supported,
> > > this means the MSI addresses need to be mapped in the IOMMU.
> > >
> > > x86 IOMMUs typically don't expose the attribute since on x86, MSI write
> > > transaction addresses always are within the 1MB PA region [FEE0_0000h -
> > > FEF0_000h] window which directly targets the APIC configuration space and
> > > hence bypass the sMMU. On ARM and PowerPC however MSI transactions are
> > > conveyed through the IOMMU.
> >
> > What's stopping us from simply inferring this from the domain's IOMMU
> > not advertising interrupt remapping capabilities?
> My current understanding is it is not possible:
> on x86 CAP_INTR_REMAP is not systematically exposed (the feature can be
> disabled) and MSIs are never mapped in the IOMMU I think.

Not sure I follow - if the feature is disabled such that the IOMMU 
doesn't isolate MSIs, then it's no different a situation from the SMMU, no?

My point was that this logic:

	if (IOMMU_CAP_INTR_REMAP)
		we're good
	else if (DOMAIN_ATTR_MSI_MAPPING)
		if (acquire_msi_remapping_resources(domain))
			we're good
		else
			oh no!
	else
		oh no!

should be easily reducible to this:

	if (IOMMU_CAP_INTR_REMAP)
		we're good
	else if (acquire_msi_remapping_resources(domain))
		we're good
	else
		oh no!	// Don't care whether the domain ran out of
			// resources or simply doesn't support it,
			// either way we can't proceed.

Robin.

> Best Regards
>
> Eric
> > Robin.
> >
> > > Signed-off-by: Bharat Bhushan <Bharat.Bhushan@xxxxxxxxxxxxx>
> > > Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx>
> > >
> > > ---
> > >
> > > v4 -> v5:
> > > - introduce the user in the next patch
> > >
> > > RFC v1 -> v1:
> > > - the data field is not used
> > > - for this attribute domain_get_attr simply returns 0 if the MSI_MAPPING
> > >     capability if needed or <0 if not.
> > > - removed struct iommu_domain_msi_maps
> > > ---
> > >    include/linux/iommu.h | 1 +
> > >    1 file changed, 1 insertion(+)
> > >
> > > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > > index 62a5eae..b3e8c5b 100644
> > > --- a/include/linux/iommu.h
> > > +++ b/include/linux/iommu.h
> > > @@ -113,6 +113,7 @@ enum iommu_attr {
> > >        DOMAIN_ATTR_FSL_PAMU_ENABLE,
> > >        DOMAIN_ATTR_FSL_PAMUV1,
> > >        DOMAIN_ATTR_NESTING,    /* two stages of translation */
> > > +    DOMAIN_ATTR_MSI_MAPPING, /* Require MSIs mapping in iommu */
> > >        DOMAIN_ATTR_MAX,
> > >    };