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

[Niklas Schnelle <schnelle@linux.ibm.com>]

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)