Re: [PATCH v5 0/2] s390x/pci: relax I/O address translation requirement

[Thomas Huth <thuth@redhat.com>]

On 26/02/2025 22.00, Matthew Rosato wrote:
> This series introduces the concept of the relaxed translation requirement
> for s390x guests in order to allow bypass of the guest IOMMU for more
> efficient PCI passthrough.
> 
> With this series, QEMU can indicate to the guest that an IOMMU is not
> strictly required for a zPCI device.  This would subsequently allow a
> guest linux to use iommu.passthrough=1 and bypass their guest IOMMU for
> PCI devices.
> 
> When this occurs, QEMU will note the behavior via an intercepted MPCIFC
> instruction and will fill the host iommu with mappings of the entire
> guest address space in response.
> 
> The kernel series [1] that adds the relevant behavior needed to
> exploit this new feature from within a s390x linux guest is available
> in linux-next via iommu-next.
> 
> [1]: https://lore.kernel.org/linux-s390/20250212213418.182902-1-mjrosato@linux.ibm.com/
> 
> Changes for v5:
> - Add some review/test tags (had to drop some due to code changes)
> - Dynamically allocate iommu->dm_mr, remove direct_map bool
> 
> Changes for v4:
> - use get_system_memory() instead of ms->ram
> - rename rtr_allowed to rtr_avail
> - turn off rtr_avail for emulated devices so MPCFIC fence properly
>    rejects an attempt at direct mapping (we only advertise via CLP
>    for passthrough devices)
> - turn off rtr_avail for passthrough ISM devices
> - various minor changes
> 
> Changes for v3:
> - use s390_get_memory_limit
> - advertise full aperture for relaxed-translation-capable devices
> 
> Changes for v2:
> - Add relax-translation property, fence for older machines
> - Add a new MPCIFC failure case when direct-mapping requested but
>    the relax-translation property is set to off.
> - For direct mapping, use a memory alias to handle the SMDA offset and
>    then just let vfio handle the pinning of memory.
> 
> Matthew Rosato (2):
>    s390x/pci: add support for guests that request direct mapping
>    s390x/pci: indicate QEMU supports relaxed translation for passthrough
> 
>   hw/s390x/s390-pci-bus.c         | 39 +++++++++++++++++++++++++++++++--
>   hw/s390x/s390-pci-inst.c        | 13 +++++++++--
>   hw/s390x/s390-pci-vfio.c        | 28 ++++++++++++++++++-----
>   hw/s390x/s390-virtio-ccw.c      |  5 +++++
>   include/hw/s390x/s390-pci-bus.h |  3 +++
>   include/hw/s390x/s390-pci-clp.h |  1 +
>   6 files changed, 80 insertions(+), 9 deletions(-)
> 

Series
Acked-by: Thomas Huth <thuth@redhat.com>

I'll queue it for my next pull request.

  Thomas