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

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

On Thu, 17 Mar 2016 19:38:29 +0800
Yongji Xie <xyjxie@linux.vnet.ibm.com> wrote:

> On 2016/3/17 0:32, Alex Williamson wrote:
> > On Mon,  7 Mar 2016 15:48:38 +0800
> > Yongji Xie <xyjxie@linux.vnet.ibm.com> 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@linux.vnet.ibm.com>
> >> ---
> >>   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