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

From: Bhushan Bharat-R65777
Date: Fri Oct 04 2013 - 12:49:46 EST

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> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Friday, October 04, 2013 9:15 PM
> To: Bhushan Bharat-R65777
> Cc: joro@xxxxxxxxxx; benh@xxxxxxxxxxxxxxxxxxx; galak@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; linuxppc-dev@xxxxxxxxxxxxxxxx; linux-
> pci@xxxxxxxxxxxxxxx; agraf@xxxxxxx; Wood Scott-B07421; iommu@xxxxxxxxxxxx
> 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@xxxxxxxxxxxxxxx
> > > [mailto:linux-pci-owner@xxxxxxxxxxxxxxx]
> > > On Behalf Of Alex Williamson
> > > Sent: Wednesday, September 25, 2013 10:16 PM
> > > To: Bhushan Bharat-R65777
> > > Cc: joro@xxxxxxxxxx; benh@xxxxxxxxxxxxxxxxxxx;
> > > galak@xxxxxxxxxxxxxxxxxxx; linux- kernel@xxxxxxxxxxxxxxx;
> > > linuxppc-dev@xxxxxxxxxxxxxxxx; linux- pci@xxxxxxxxxxxxxxx;
> > > agraf@xxxxxxx; Wood Scott-B07421; iommu@xxxxxxxxxxxx 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@xxxxxxxxxxxxx>
> > > > ---
> > > > 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;
> > >
> > >
> > >
> > > --
> > > To unsubscribe from this list: send the line "unsubscribe linux-pci"
> > > in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo
> > > info at http://vger.kernel.org/majordomo-info.html
> >
>
>
>

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Next message: Peter Zijlstra: "Re: tty^Wrcu/perf lockdep trace."
Previous message: Olof Johansson: "Re: Use of drivers/platform and matching include?"
In reply to: Alex Williamson: "Re: [PATCH 2/7] iommu: add api to get iommu_domain of a device"
Next in thread: Alex Williamson: "Re: [PATCH 2/7] iommu: add api to get iommu_domain of a device"
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