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

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

On Mon, 2013-10-14 at 12:58 +0000, Sethi Varun-B16395 wrote:
> 
> > -----Original Message-----
> > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > Sent: Friday, October 11, 2013 2:12 AM
> > To: Sethi Varun-B16395
> > Cc: Bhushan Bharat-R65777; agraf@suse.de; Wood Scott-B07421; linux-
> > pci@vger.kernel.org; galak@kernel.crashing.org; linux-
> > kernel@vger.kernel.org; iommu@lists.linux-foundation.org;
> > benh@kernel.crashing.org; linuxppc-dev@lists.ozlabs.org
> > Subject: Re: [PATCH 2/7] iommu: add api to get iommu_domain of a device
> > 
> > On Thu, 2013-10-10 at 20:09 +0000, Sethi Varun-B16395 wrote:
> > >
> > > > -----Original Message-----
> > > > From: iommu-bounces@lists.linux-foundation.org [mailto:iommu-
> > > > bounces@lists.linux-foundation.org] On Behalf Of Alex Williamson
> > > > Sent: Tuesday, October 08, 2013 8:43 AM
> > > > To: Bhushan Bharat-R65777
> > > > Cc: agraf@suse.de; Wood Scott-B07421; linux-pci@vger.kernel.org;
> > > > galak@kernel.crashing.org; linux-kernel@vger.kernel.org;
> > > > iommu@lists.linux-foundation.org; benh@kernel.crashing.org;
> > > > linuxppc- dev@lists.ozlabs.org
> > > > Subject: Re: [PATCH 2/7] iommu: add api to get iommu_domain of a
> > > > device
> > > >
> > > > 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.
> > > [Sethi Varun-B16395] What you are suggesting is that the MSI setup
> > > path queries the vfio subsystem for the mapping, rather than directly
> > > querying the iommu subsystem. So, say if we add an interface for
> > > getting MSI to IOVA mapping in the msi setup path, wouldn't this again
> > > be specific to vfio? I reckon that this interface would again ppc
> > > machine specific interface.
> > 
> > Sure, if this is a generic MSI setup path then clearly vfio should not be
> > involved.  But by that same notion, if this is a generic MSI setup path,
> > how can the proposed solutions guarantee that the iommu_domain or iova
> > returned is still valid in all cases?  It's racy.  If the caller trying
> > to setup MSI has the information needed, why doesn't it pass it in as
> > part of the setup?  Thanks,
> [Sethi Varun-B16395] Agreed, the proposed interface is not clean. But, we still need an interface through which MSI driver can call in to the vfio layer.

Or one that allows vfio to pass in the iova when the interrupt is being
setup.  I'm afraid any kind of reverse interface where MSI calls back
into vfio is going to be racy.  Thanks,

Alex