Re: [PATCH 2/4] iommu/arm-smmu-v3: Rework enabling/disabling of ATS for PCI masters

[Will Deacon <will@kernel.org>]

On Tue, Aug 20, 2019 at 05:12:11PM +0100, Robin Murphy wrote:
> On 20/08/2019 16:45, Will Deacon wrote:
> > To prevent any potential issues arising from speculative Address
> > Translation Requests from an ATS-enabled PCIe endpoint, rework our ATS
> > enabling/disabling logic so that we enable ATS at the SMMU before we
> > enable it at the endpoint, and disable things in the opposite order.
> > 
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >   drivers/iommu/arm-smmu-v3.c | 44 ++++++++++++++++++++++++++------------------
> >   1 file changed, 26 insertions(+), 18 deletions(-)
> > 
> > diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> > index 3402b1bc8e94..9096eca0c480 100644
> > --- a/drivers/iommu/arm-smmu-v3.c
> > +++ b/drivers/iommu/arm-smmu-v3.c
> > @@ -2283,31 +2283,34 @@ static void arm_smmu_install_ste_for_dev(struct arm_smmu_master *master)
> >   	}
> >   }
> > -static int arm_smmu_enable_ats(struct arm_smmu_master *master)
> > +static bool arm_smmu_ats_supported(struct arm_smmu_master *master)
> >   {
> > -	int ret;
> > -	size_t stu;
> >   	struct pci_dev *pdev;
> >   	struct arm_smmu_device *smmu = master->smmu;
> >   	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(master->dev);
> >   	if (!(smmu->features & ARM_SMMU_FEAT_ATS) || !dev_is_pci(master->dev) ||
> >   	    !(fwspec->flags & IOMMU_FWSPEC_PCI_RC_ATS) || pci_ats_disabled())
> > -		return -ENXIO;
> > +		return false;
> >   	pdev = to_pci_dev(master->dev);
> > -	if (pdev->untrusted)
> > -		return -EPERM;
> > +	return !pdev->untrusted && pdev->ats_cap;
> > +}
> > -	/* Smallest Translation Unit: log2 of the smallest supported granule */
> > -	stu = __ffs(smmu->pgsize_bitmap);
> > +static void arm_smmu_enable_ats(struct arm_smmu_master *master)
> > +{
> > +	size_t stu;
> > +	struct pci_dev *pdev;
> > +	struct arm_smmu_device *smmu = master->smmu;
> > -	ret = pci_enable_ats(pdev, stu);
> > -	if (ret)
> > -		return ret;
> > +	if (master->ats_enabled || !dev_is_pci(master->dev))
> > +		return;
> > -	master->ats_enabled = true;
> > -	return 0;
> > +	/* Smallest Translation Unit: log2 of the smallest supported granule */
> > +	stu = __ffs(smmu->pgsize_bitmap);
> > +	pdev = to_pci_dev(master->dev);
> > +	if (pci_enable_ats(pdev, stu))
> > +		dev_err(master->dev, "Failed to enable ATS (STU %zu)\n", stu);
> >   }
> >   static void arm_smmu_disable_ats(struct arm_smmu_master *master)
> > @@ -2317,10 +2320,14 @@ static void arm_smmu_disable_ats(struct arm_smmu_master *master)
> >   	if (!master->ats_enabled || !dev_is_pci(master->dev))
> 
> Hmm, while you've got the lid off, that dev_is_pci() test is clearly
> redundant.

Good point; I'll kill it.

> >   		return;
> > +	pci_disable_ats(to_pci_dev(master->dev));
> > +	/*
> > +	 * Ensure ATS is disabled at the endpoint before we issue the
> > +	 * ATC invalidation via the SMMU.
> > +	 */
> > +	wmb();
> >   	arm_smmu_atc_inv_to_cmd(0, 0, 0, &cmd);
> >   	arm_smmu_atc_inv_master(master, &cmd);
> > -	pci_disable_ats(to_pci_dev(master->dev));
> > -	master->ats_enabled = false;
> >   }
> >   static void arm_smmu_detach_dev(struct arm_smmu_master *master)
> > @@ -2335,10 +2342,10 @@ static void arm_smmu_detach_dev(struct arm_smmu_master *master)
> >   	list_del(&master->domain_head);
> >   	spin_unlock_irqrestore(&smmu_domain->devices_lock, flags);
> > +	arm_smmu_disable_ats(master);
> >   	master->domain = NULL;
> > +	master->ats_enabled = false;
> >   	arm_smmu_install_ste_for_dev(master);
> > -
> > -	arm_smmu_disable_ats(master);
> >   }
> >   static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
> > @@ -2383,12 +2390,13 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
> >   	spin_unlock_irqrestore(&smmu_domain->devices_lock, flags);
> >   	if (smmu_domain->stage != ARM_SMMU_DOMAIN_BYPASS)
> > -		arm_smmu_enable_ats(master);
> > +		master->ats_enabled = arm_smmu_ats_supported(master);
> 
> So for non-bypass domains we pretend ATS is already enabled iff it could
> possibly be...
> 
> >   	if (smmu_domain->stage == ARM_SMMU_DOMAIN_S1)
> >   		arm_smmu_write_ctx_desc(smmu, &smmu_domain->s1_cfg);
> >   	arm_smmu_install_ste_for_dev(master);
> > +	arm_smmu_enable_ats(master);
> 
> ...which ensures this won't actually touch the PCIe cap, unless of course
> when STE.EATS == 0. Are you sure about that?

Argh, too many "ats_enabled" flags! (there's another one in the pci
device). I should probably invert the check, but let me have a play --
the idea is that arm_smmu_master::ats_enabled is initially used to
configure the STE and then acts as a proxy for the STE state after that.

Thanks for the review.

Will
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