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


> -----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.

-Varun


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