Re: [PATCH 1/1] iommu/vt-d: Support asynchronous IOMMU nested capabilities

[Lu Baolu]

On 5/13/21 10:26 AM, Tian, Kevin wrote:
> > From: Lu Baolu
> > Sent: Wednesday, May 12, 2021 7:31 PM
> >
> > Hi Kevin,
> >
> > On 5/12/21 4:30 PM, Tian, Kevin wrote:
> > > > From: Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>
> > > > Sent: Wednesday, May 12, 2021 3:04 PM
> > > >
> > > > Current VT-d implementation supports nested translation only if all
> > > > underlying IOMMUs support the nested capability. This is unnecessary
> > > > as the upper layer is allowed to create different containers and set
> > > > them with different type of iommu backend. The IOMMU driver needs to
> > > > guarantee that devices attached to a nested mode iommu_domain should
> > > > support nested capabilility.
> > > so the consistency check is now applied only to the IOMMUs that are
> > > spanned by a given iommu_domain?
> > Yes.
> >
> > > > Signed-off-by: Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>
> > > > ---
> > > >    drivers/iommu/intel/iommu.c | 21 +++++++++++++++++++--
> > > >    1 file changed, 19 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> > > > index f1742da42478..1cd4840e6f9f 100644
> > > > --- a/drivers/iommu/intel/iommu.c
> > > > +++ b/drivers/iommu/intel/iommu.c
> > > > @@ -4755,6 +4755,13 @@ static int
> > prepare_domain_attach_device(struct
> > > > iommu_domain *domain,
> > > >    	if (!iommu)
> > > >    		return -ENODEV;
> > > >
> > > > +	if ((dmar_domain->flags & DOMAIN_FLAG_NESTING_MODE) &&
> > > > +	    !ecap_nest(iommu->ecap)) {
> > > > +		dev_err(dev, "%s: iommu not support nested translation\n",
> > > > +			iommu->name);
> > > > +		return -EINVAL;
> > > > +	}
> > > > +
> > > >    	/* check if this iommu agaw is sufficient for max mapped address */
> > > >    	addr_width = agaw_to_width(iommu->agaw);
> > > >    	if (addr_width > cap_mgaw(iommu->cap))
> > > > @@ -5451,11 +5458,21 @@ static int
> > > >    intel_iommu_enable_nesting(struct iommu_domain *domain)
> > > >    {
> > > >    	struct dmar_domain *dmar_domain = to_dmar_domain(domain);
> > > > +	struct dmar_drhd_unit *drhd;
> > > > +	struct intel_iommu *iommu;
> > > > +	bool has_nesting = false;
> > > >    	unsigned long flags;
> > > > -	int ret = -ENODEV;
> > > > +	int ret = -EINVAL;
> > > > +
> > > > +	for_each_active_iommu(iommu, drhd)
> > > > +		if (ecap_nest(iommu->ecap))
> > > > +			has_nesting = true;
> > > > +
> > > > +	if (!has_nesting)
> > > > +		return -ENODEV;
> > > Isn't above still doing global consistency check?
> > The logic is if nested mode is globally unsupported, return false.
> why is this check required? anyway you already have the check when
> prepare device attachment, and only at that point you have accurate
> info for which iommu to be checked. This check looks meaningless
> as even if some IOMMUs support nesting it doesn't mean the IOMMU
> relevant to this domain support it.

Get you. It's really unnecessary. I will drop this check in the new
version.

Best regards,
baolu