[PATCH 2/7] iommu: add api to get iommu_domain of a device

[Bhushan Bharat-R65777]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson at redhat.com]
> Sent: Friday, October 04, 2013 9:15 PM
> To: Bhushan Bharat-R65777
> Cc: joro at 8bytes.org; benh at kernel.crashing.org; galak at kernel.crashing.org; linux-
> kernel at vger.kernel.org; linuxppc-dev at lists.ozlabs.org; linux-
> pci at vger.kernel.org; agraf at suse.de; Wood Scott-B07421; iommu at lists.linux-
> foundation.org
> Subject: Re: [PATCH 2/7] iommu: add api to get iommu_domain of a device
> 
> On Fri, 2013-10-04 at 09:54 +0000, Bhushan Bharat-R65777 wrote:
> >
> > > -----Original Message-----
> > > From: linux-pci-owner at vger.kernel.org
> > > [mailto:linux-pci-owner at vger.kernel.org]
> > > On Behalf Of Alex Williamson
> > > Sent: Wednesday, September 25, 2013 10:16 PM
> > > To: Bhushan Bharat-R65777
> > > Cc: joro at 8bytes.org; benh at kernel.crashing.org;
> > > galak at kernel.crashing.org; linux- kernel at vger.kernel.org;
> > > linuxppc-dev at lists.ozlabs.org; linux- pci at vger.kernel.org;
> > > agraf at suse.de; Wood Scott-B07421; iommu at lists.linux- foundation.org;
> > > Bhushan Bharat-R65777
> > > Subject: Re: [PATCH 2/7] iommu: add api to get iommu_domain of a
> > > device
> > >
> > > On Thu, 2013-09-19 at 12:59 +0530, Bharat Bhushan wrote:
> > > > This api return the iommu domain to which the device is attached.
> > > > The iommu_domain is required for making API calls related to iommu.
> > > > Follow up patches which use this API to know iommu maping.
> > > >
> > > > Signed-off-by: Bharat Bhushan <bharat.bhushan at freescale.com>
> > > > ---
> > > >  drivers/iommu/iommu.c |   10 ++++++++++
> > > >  include/linux/iommu.h |    7 +++++++
> > > >  2 files changed, 17 insertions(+), 0 deletions(-)
> > > >
> > > > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c index
> > > > fbe9ca7..6ac5f50 100644
> > > > --- a/drivers/iommu/iommu.c
> > > > +++ b/drivers/iommu/iommu.c
> > > > @@ -696,6 +696,16 @@ void iommu_detach_device(struct iommu_domain
> > > > *domain, struct device *dev)  }
> > > > EXPORT_SYMBOL_GPL(iommu_detach_device);
> > > >
> > > > +struct iommu_domain *iommu_get_dev_domain(struct device *dev) {
> > > > +	struct iommu_ops *ops = dev->bus->iommu_ops;
> > > > +
> > > > +	if (unlikely(ops == NULL || ops->get_dev_iommu_domain == NULL))
> > > > +		return NULL;
> > > > +
> > > > +	return ops->get_dev_iommu_domain(dev); }
> > > > +EXPORT_SYMBOL_GPL(iommu_get_dev_domain);
> > >
> > > What prevents this from racing iommu_domain_free()?  There's no
> > > references acquired, so there's no reason for the caller to assume the
> pointer is valid.
> >
> > Sorry for late query, somehow this email went into a folder and
> > escaped;
> >
> > Just to be sure, there is not lock at generic "struct iommu_domain", but IP
> specific structure (link FSL domain) linked in iommu_domain->priv have a lock,
> so we need to ensure this race in FSL iommu code (say
> drivers/iommu/fsl_pamu_domain.c), right?
> 
> No, it's not sufficient to make sure that your use of the interface is race
> free.  The interface itself needs to be designed so that it's difficult to use
> incorrectly.

So we can define iommu_get_dev_domain()/iommu_put_dev_domain();  iommu_get_dev_domain() will return domain with the lock held, and iommu_put_dev_domain() will release the lock? And iommu_get_dev_domain() must always be followed by iommu_get_dev_domain().


> That's not the case here.  This is a backdoor to get the iommu
> domain from the iommu driver regardless of who is using it or how.  The iommu
> domain is created and managed by vfio, so shouldn't we be looking at how to do
> this through vfio?

Let me first describe what we are doing here:
During initialization:-
 - vfio talks to MSI system to know the MSI-page and size
 - vfio then interacts with iommu to map the MSI-page in iommu (IOVA is decided by userspace and physical address is the MSI-page)
 - So the IOVA subwindow mapping is created in iommu and yes VFIO know about this mapping.

Now do SET_IRQ(MSI/MSIX) ioctl:
 - calls pci_enable_msix()/pci_enable_msi_block(): which is supposed to set MSI address/data in device.
 - So in current implementation (this patchset) msi-subsystem gets the IOVA from iommu via this defined interface.
 - Are you saying that rather than getting this from iommu, we should get this from vfio? What difference does this make?

Thanks
-Bharat

> It seems like you'd want to use your device to get a vfio
> group reference, from which you could do something with the vfio external user
> interface and get the iommu domain reference.  Thanks,
> 
> Alex
> 
> > > >  /*
> > > >   * IOMMU groups are really the natrual working unit of the IOMMU, but
> > > >   * the IOMMU API works on domains and devices.  Bridge that gap
> > > > by diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > > > index 7ea319e..fa046bd 100644
> > > > --- a/include/linux/iommu.h
> > > > +++ b/include/linux/iommu.h
> > > > @@ -127,6 +127,7 @@ struct iommu_ops {
> > > >  	int (*domain_set_windows)(struct iommu_domain *domain, u32
> w_count);
> > > >  	/* Get the numer of window per domain */
> > > >  	u32 (*domain_get_windows)(struct iommu_domain *domain);
> > > > +	struct iommu_domain *(*get_dev_iommu_domain)(struct device
> > > > +*dev);
> > > >
> > > >  	unsigned long pgsize_bitmap;
> > > >  };
> > > > @@ -190,6 +191,7 @@ extern int iommu_domain_window_enable(struct
> > > > iommu_domain
> > > *domain, u32 wnd_nr,
> > > >  				      phys_addr_t offset, u64 size,
> > > >  				      int prot);
> > > >  extern void iommu_domain_window_disable(struct iommu_domain
> > > > *domain,
> > > > u32 wnd_nr);
> > > > +extern struct iommu_domain *iommu_get_dev_domain(struct device
> > > > +*dev);
> > > >  /**
> > > >   * report_iommu_fault() - report about an IOMMU fault to the IOMMU
> framework
> > > >   * @domain: the iommu domain where the fault has happened @@
> > > > -284,6
> > > > +286,11 @@ static inline void iommu_domain_window_disable(struct
> > > > iommu_domain *domain,  {  }
> > > >
> > > > +static inline struct iommu_domain *iommu_get_dev_domain(struct
> > > > +device
> > > > +*dev) {
> > > > +	return NULL;
> > > > +}
> > > > +
> > > >  static inline phys_addr_t iommu_iova_to_phys(struct iommu_domain
> > > > *domain, dma_addr_t iova)  {
> > > >  	return 0;
> > >
> > >
> > >
> > > --
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> >
> 
> 
>