Re: [PATCH v3 3/3] iommu/s390: implement iommu passthrough via identity domain

Niklas Schnelle
2025-01-30T07:43:59Z

On Fri, 2025-01-24 at 15:17 -0500, Matthew Rosato wrote:
> Enabled via the kernel command-line 'iommu.passthrough=1' option.
>
> Introduce the concept of identity domains to s390-iommu, which relies on
> the bus_dma_region to offset identity mappings to the start of the DMA
> aperture advertized by CLP.
>
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> ---
> arch/s390/pci/pci.c | 6 ++-
> drivers/iommu/s390-iommu.c | 95 +++++++++++++++++++++++++++++---------
> 2 files changed, 76 insertions(+), 25 deletions(-)
>
> diff --git a/arch/s390/pci/pci.c b/arch/s390/pci/pci.c
> index 88f72745fa59..758b23331754 100644
> --- a/arch/s390/pci/pci.c
> +++ b/arch/s390/pci/pci.c
> @@ -124,14 +124,16 @@ int zpci_register_ioat(struct zpci_dev *zdev, u8 dmaas,
> struct zpci_fib fib = {0};
> u8 cc;
>
> - WARN_ON_ONCE(iota & 0x3fff);
> fib.pba = base;
> /* Work around off by one in ISM virt device */
> if (zdev->pft == PCI_FUNC_TYPE_ISM && limit > base)
> fib.pal = limit + (1 << 12);
> else
> fib.pal = limit;
> - fib.iota = iota | ZPCI_IOTA_RTTO_FLAG;
> + if (iota == 0)
> + fib.iota = iota;

Taking another look, I think there is a small problem with the logic of
passing iota == 0 to indicate direct mapping. In
zpci_hot_reset_device() we call zpci_register_ioat() with iota set to
virt_to_phys(zdev->dma_table) and while zdev->dma_table is NULL for the
identity domain we can't rely on virt_to_phys(NULL) == NULL.

> + else
> + fib.iota = iota | ZPCI_IOTA_RTTO_FLAG;
> fib.gd = zdev->gisa;
> cc = zpci_mod_fc(req, &fib, status);
> if (cc)