Re: [RFC PATCH v4 7/7] powerpc/powernv/pci-ioda: Add IOMMU_CAP_INTR_REMAP for IODA host bridge

[Yongji Xie]

On 2016/3/17 20:48, Alex Williamson wrote:
> On Thu, 17 Mar 2016 19:38:29 +0800
> Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx> wrote:
>
> > On 2016/3/17 0:32, Alex Williamson wrote:
> > > On Mon,  7 Mar 2016 15:48:38 +0800
> > > Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx> wrote:
> > >   
> > > > This patch adds IOMMU_CAP_INTR_REMAP for IODA host bridge so that
> > > > we can mmap MSI-X table in vfio driver.
> > > >
> > > > Signed-off-by: Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx>
> > > > ---
> > > >    arch/powerpc/platforms/powernv/pci-ioda.c |   17 +++++++++++++++++
> > > >    1 file changed, 17 insertions(+)
> > > >
> > > > diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c b/arch/powerpc/platforms/powernv/pci-ioda.c
> > > > index f90dc04..f01b9ab 100644
> > > > --- a/arch/powerpc/platforms/powernv/pci-ioda.c
> > > > +++ b/arch/powerpc/platforms/powernv/pci-ioda.c
> > > > @@ -1955,6 +1955,20 @@ static struct iommu_table_ops pnv_ioda2_iommu_ops = {
> > > >    	.free = pnv_ioda2_table_free,
> > > >    };
> > > >    
> > > > +static bool pnv_ioda_iommu_capable(enum iommu_cap cap)
> > > > +{
> > > > +	switch (cap) {
> > > > +	case IOMMU_CAP_INTR_REMAP:
> > > > +		return true;
> > > > +	default:
> > > > +		return false;
> > > > +	}
> > > > +}
> > > > +
> > > > +static struct iommu_ops pnv_ioda_iommu_ops = {
> > > > +	.capable = pnv_ioda_iommu_capable,
> > > > +};
> > > > +
> > > >    static void pnv_pci_ioda_setup_dma_pe(struct pnv_phb *phb,
> > > >    				      struct pnv_ioda_pe *pe, unsigned int base,
> > > >    				      unsigned int segs)
> > > > @@ -3078,6 +3092,9 @@ static void pnv_pci_ioda_fixup(void)
> > > >    
> > > >    	/* Link NPU IODA tables to their PCI devices. */
> > > >    	pnv_npu_ioda_fixup();
> > > > +
> > > > +	/* Add IOMMU_CAP_INTR_REMAP */
> > > > +	bus_set_iommu(&pci_bus_type, &pnv_ioda_iommu_ops);
> > > >    }
> > > >    
> > > >    /*
> > > Doesn't this set you up for a world of hurt?  bus_set_iommu() calls
> > > iommu_bus_init() which sets up notifiers, which maybe you don't care
> > > about, but it also means that iommu_domain_alloc(&pci_bus_type) will
> > > segfault because you're not providing a domain_alloc callback here.
> > It seems to be hard to add IOMMU_CAP_INTR_REMAP on
> > PPC64 platform.
> >
> > And can we add a new ioctl in vfio_iommu_driver to check
> > if interrupt remapping is supported so that we can use our
> > own way to determine that on PPC64 platform?
> I'd prefer not.  At the vfio user API level, the question is whether
> the user can mmap over the msix table, testing a property/ioctl on the
> iommu driver seems like an odd way to discover that.  We should be
> determining whether that's safe in the kernel and exporting that info
> on the vfio device itself, where it seems like we have various ways we
> could do this within the existing ioctls.  Thanks,
>
> Alex

Yes, you are right. It's not a good idea to add a new ioctl in
vfio_iommu_driver. Now I'd like to talk about the way to
determining whether it's safe to mmap over the msix table.

We currently use IOMMU_CAP_INTR_REMAP to determine that.
But there are some problems on PPC64 which never set
iommu_ops and ARM SMMU which set this capability but not
provide interrupt isolation. Can we add a variable/property
which can be set in vfio_iommu_driver->ops->attach_group()
and used in vfio_pci_driver to determine whether we can allow
mmapping msix table?  If so, we can still use
IOMMU_CAP_INTR_REMAP, or some arch-independent ways
when IOMMU_CAP_INTR_REMAP doesn't work.

Thanks,
Yongji Xie