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

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 2013-10-07 at 05:46 +0000, Bhushan Bharat-R65777 wrote:
> 
> > -----Original Message-----
> > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > Sent: Friday, October 04, 2013 11:42 PM
> > To: Bhushan Bharat-R65777
> > Cc: joro@8bytes.org; benh@kernel.crashing.org; galak@kernel.crashing.org; linux-
> > kernel@vger.kernel.org; linuxppc-dev@lists.ozlabs.org; linux-
> > pci@vger.kernel.org; agraf@suse.de; Wood Scott-B07421; iommu@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 17:23 +0000, Bhushan Bharat-R65777 wrote:
> > >
> > > > -----Original Message-----
> > > > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > > > Sent: Friday, October 04, 2013 10:43 PM
> > > > To: Bhushan Bharat-R65777
> > > > Cc: joro@8bytes.org; benh@kernel.crashing.org;
> > > > galak@kernel.crashing.org; linux- kernel@vger.kernel.org;
> > > > linuxppc-dev@lists.ozlabs.org; linux- pci@vger.kernel.org;
> > > > agraf@suse.de; Wood Scott-B07421; iommu@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 16:47 +0000, Bhushan Bharat-R65777 wrote:
> > > > >
> > > > > > -----Original Message-----
> > > > > > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > > > > > Sent: Friday, October 04, 2013 9:15 PM
> > > > > > To: Bhushan Bharat-R65777
> > > > > > Cc: joro@8bytes.org; benh@kernel.crashing.org;
> > > > > > galak@kernel.crashing.org; linux- kernel@vger.kernel.org;
> > > > > > linuxppc-dev@lists.ozlabs.org; linux- pci@vger.kernel.org;
> > > > > > agraf@suse.de; Wood Scott-B07421; iommu@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@vger.kernel.org
> > > > > > > > [mailto:linux-pci-owner@vger.kernel.org]
> > > > > > > > On Behalf Of Alex Williamson
> > > > > > > > Sent: Wednesday, September 25, 2013 10:16 PM
> > > > > > > > To: Bhushan Bharat-R65777
> > > > > > > > Cc: joro@8bytes.org; benh@kernel.crashing.org;
> > > > > > > > galak@kernel.crashing.org; linux- kernel@vger.kernel.org;
> > > > > > > > linuxppc-dev@lists.ozlabs.org; linux- pci@vger.kernel.org;
> > > > > > > > agraf@suse.de; Wood Scott-B07421; iommu@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@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().
> > > >
> > > > What lock?  get/put are generally used for reference counting, not
> > > > locking in the kernel.
> > > >
> > > > > > 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?
> > > >
> > > > Yes, you just said above that vfio knows the msi to iova mapping, so
> > > > why go outside of vfio to find it later?  The difference is one case
> > > > you can have a proper reference to data structures to make sure the
> > > > pointer you get back actually has meaning at the time you're using
> > > > it vs the code here where you're defining an API that returns a
> > > > meaningless value
> > >
> > > With FSL-PAMU we will always get consistant data from iommu or vfio-data
> > structure.
> > 
> > Great, but you're trying to add a generic API to the IOMMU subsystem that's
> > difficult to use correctly.  The fact that you use it correctly does not justify
> > the API.
> > 
> > > > because you can't check or
> > > > enforce that an arbitrary caller is using it correctly.
> > >
> > > I am not sure what is arbitrary caller? pdev is known to vfio, so vfio
> > > will only make pci_enable_msix()/pci_enable_msi_block() for this pdev.
> > > If anyother code makes then it is some other unexpectedly thing
> > > happening in system, no?
> > 
> > What's proposed here is a generic IOMMU API.  Anybody can call this.
> > What if the host SCSI driver decides to go get the iommu domain for it's device
> > (or any other device)?  Does that fit your usage model?
> > 
> > > >  It's not maintainable.
> > > > Thanks,
> > >
> > > I do not have any issue with this as well, can you also describe the
> > > type of API you are envisioning; I can think of defining some function
> > > in vfio.c/vfio_iommu*.c, make them global and declare then in
> > > include/Linux/vfio.h And include <Linux/vfio.h> in caller file
> > > (arch/powerpc/kernel/msi.c)
> > 
> > Do you really want module dependencies between vfio and your core kernel MSI
> > setup?  Look at the vfio external user interface that we've already defined.
> > That allows other components of the kernel to get a proper reference to a vfio
> > group.  From there you can work out how to get what you want.  Another
> > alternative is that vfio could register an MSI to IOVA mapping with architecture
> > code when the mapping is created.  The MSI setup path could then do a lookup in
> > architecture code for the mapping.  You could even store the MSI to IOVA mapping
> > in VFIO and create an interface where SET_IRQ passes that mapping into setup
> > code.
> 
> Ok, What I want is to get IOVA associated with a physical address
> (physical address of MSI-bank).
> And currently I do not see a way to know IOVA of a physical address
> and doing all this domain get and then search through all of
> iommu-windows of that domain.
> 
> What if we add an iommu-API which can return the IOVA mapping of a
> physical address. Current use case is setting up MSI's for aperture
> type of IOMMU also getting a phys_to_iova() mapping is independent of
> VFIO, your thought?

A physical address can be mapped to multiple IOVAs, so the interface
seems flawed by design.  It also has the same problem as above, it's a
backdoor that can be called asynchronous to the owner of the domain, so
what reason is there to believe the result?  It just replaces an
iommu_domain pointer with an IOVA.  VFIO knows this mapping, so why are
we trying to go behind its back and ask the IOMMU?  Thanks,

Alex