Re: [PATCH] iommu/vt-d: Handle hotplug devices' default identity mapping setting

[Lu Baolu]

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Hi James,

On 2/22/19 3:36 PM, James Dong wrote:
> Baolu:
>
> Sorry that my last reply email seems not text format. Resend it now.
>
> Thanks for your comments and your patch. Please find below our responses to
> each of your comments:
>
> > What does "I/O operation won't work" exactly mean here? Do you see any
> > IOMMU fault message? Or, something doesn't work as expected?
>
> Yes, DMAR fault messages as following came out:
> [  354.939896] DMAR: DMAR:[DMA Read] Request device [03:00.1]fault addr 1fdfe80000
> [  354.939896] DMAR:[fault reason 02] Present bit in context entry is clear
>
>
> > Do you mind checking this?
> >
> > index 6ecdcf8fc8c0..f62f30bc1339 100644
> > --- a/drivers/iommu/intel-iommu.c
> > +++ b/drivers/iommu/intel-iommu.c
> > @@ -2632,6 +2632,9 @@ static struct dmar_domain
> > *find_or_alloc_domain(struct device *dev, int gaw)
> >                           goto out;
> >           }
> >
> > +       if (!iommu_should_identity_map(dev, 0))
> > +               return si_domain;
> > +
> >           /* Allocate and initialize new domain for the device */
> >           domain = alloc_domain(0);
> >           if (!domain)
>
> Tried this patch, and the same DMAR fault message came out.

Thank you!

> Guess it is because of the iommu code path for hotplug devices. If a hotplug
> device is rescanned after removal, iommu_bus_notifier will be called as part
> of the notifier chains to handle BUS_NOTIFY_ADD_DEVICE event. Along the code
> path, intel_iommu_ops->add_device() created an iommu group for this hotplug
> device, but failed to create an iommu domain because of the default domain
> type IOMMU_DOMAIN_IDENTITY imposed by current IOMMU command line option got
> declined by intel_iommu_ops->domain_alloc().

The Intel IOMMU driver hasn't switched to default domain yet although
it's in the pipe line. So, there should be no domain allocated when a
group is allocated for the device.

The problem is we need to check whether a hot-added device requires
identity map instead of allocating a normal domain blindly.

> Since si_domain is type of "struct dmar_domain", which is platform dependent,
> it is hard to make this change in intel_iommu_ops->domain_alloc().
>
> In your patch, function find_or_alloc_domain() is not in the code path of
> BUS_NOTIFY_ADD_DEVICE event notifier chain.
>
> Please let us know if your have more concerns and suggestions.

Can you please try the patch attached? I think this is a generic issue
as I described in the commit message.

Best regards,
Lu Baolu