Re: [PATCH v3 1/2] s390x/pci: add support for guests that request direct mapping

[Niklas Schnelle]

On Fri, 2025-01-24 at 15:21 -0500, Matthew Rosato wrote:
> When receiving a guest mpcifc(4) or mpcifc(6) instruction without the T
> bit set, treat this as a request to perform direct mapping instead of
> address translation.  In order to facilitate this, pin the entirety of
> guest memory into the host iommu.
> 
> Pinning for the direct mapping case is handled via vfio and its memory
> listener.  Additionally, ram discard settings are inherited from vfio:
> coordinated discards (e.g. virtio-mem) are allowed while uncoordinated
> discards (e.g. virtio-balloon) are disabled.
> 
> Subsequent guest DMA operations are all expected to be of the format
> guest_phys+sdma, allowing them to be used as lookup into the host
> iommu table.
> 
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> ---
>  hw/s390x/s390-pci-bus.c         | 36 +++++++++++++++++++++++++++++++--
>  hw/s390x/s390-pci-inst.c        | 13 ++++++++++--
>  hw/s390x/s390-pci-vfio.c        | 15 ++++++++++----
>  hw/s390x/s390-virtio-ccw.c      |  5 +++++
>  include/hw/s390x/s390-pci-bus.h |  4 ++++
>  5 files changed, 65 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
> index eead269cc2..8cf5aec1a2 100644
> --- a/hw/s390x/s390-pci-bus.c
> +++ b/hw/s390x/s390-pci-bus.c
> @@ -18,6 +18,8 @@
>  #include "hw/s390x/s390-pci-inst.h"
>  #include "hw/s390x/s390-pci-kvm.h"
>  #include "hw/s390x/s390-pci-vfio.h"
> +#include "hw/s390x/s390-virtio-ccw.h"
> +#include "hw/boards.h"
>  #include "hw/pci/pci_bus.h"
>  #include "hw/qdev-properties.h"
>  #include "hw/pci/pci_bridge.h"
> @@ -720,16 +722,44 @@ void s390_pci_iommu_enable(S390PCIIOMMU *iommu)
>                               TYPE_S390_IOMMU_MEMORY_REGION, 
> OBJECT(&iommu->mr),
>                               name, iommu->pal + 1);
>      iommu->enabled = true;
> +    iommu->direct_map = false;
>      memory_region_add_subregion(&iommu->mr, 0, 
> MEMORY_REGION(&iommu->iommu_mr));
>      g_free(name);
>  }
>  
> +void s390_pci_iommu_dm_enable(S390PCIIOMMU *iommu)
> +{
> +    MachineState *ms = MACHINE(qdev_get_machine());
> +    S390CcwMachineState *s390ms = S390_CCW_MACHINE(ms);
> +
> +    /*
> +     * For direct-mapping we must map the entire guest address space.  Rather
> +     * than using an iommu, create a memory region alias that maps GPA X to
> +     * iova X + SDMA.  VFIO will handle pinning via its memory listener.

Nit: I'd capitalize IOVA to match GPA.

> +     */
> +    g_autofree char *name = g_strdup_printf("iommu-dm-s390-%04x",
> +                                            iommu->pbdev->uid);
> +    memory_region_init_alias(&iommu->dm_mr, OBJECT(&iommu->mr), name, 
> ms->ram,
> +                             0, s390_get_memory_limit(s390ms));
> +    iommu->enabled = true;
> +    iommu->direct_map = true;
> +    memory_region_add_subregion(&iommu->mr, iommu->pbdev->zpci_fn.sdma,
> +                                &iommu->dm_mr);
> +}
> +
>  void s390_pci_iommu_disable(S390PCIIOMMU *iommu)
>  {
>      iommu->enabled = false;
>      g_hash_table_remove_all(iommu->iotlb);
> -    memory_region_del_subregion(&iommu->mr, MEMORY_REGION(&iommu->iommu_mr));
> -    object_unparent(OBJECT(&iommu->iommu_mr));
> +    if (iommu->direct_map) {
> +        memory_region_del_subregion(&iommu->mr, &iommu->dm_mr);
> +        iommu->direct_map = false;
> +        object_unparent(OBJECT(&iommu->dm_mr));
> +    } else {
> +        memory_region_del_subregion(&iommu->mr,
> +                                    MEMORY_REGION(&iommu->iommu_mr));
> +        object_unparent(OBJECT(&iommu->iommu_mr));
> +    }
>  }
>  
>  static void s390_pci_iommu_free(S390pciState *s, PCIBus *bus, int32_t devfn)
> @@ -1488,6 +1518,8 @@ static const Property s390_pci_device_properties[] = {
>      DEFINE_PROP_BOOL("interpret", S390PCIBusDevice, interp, true),
>      DEFINE_PROP_BOOL("forwarding-assist", S390PCIBusDevice, 
> forwarding_assist,
>                       true),
> +    DEFINE_PROP_BOOL("relaxed-translation", S390PCIBusDevice, rtr_allowed,
> +                     true),

Question: Do we maybe want to default rtr_allowed to false for ISM
devices? Performance wise it doesn't matter much since they keep their
mappings fairly static and it would help us catch bugs in the handling
of rtr_allowed == false devices, the ISM driver and increase security.

>  };
>  
>  static const VMStateDescription s390_pci_device_vmstate = {
> diff --git a/hw/s390x/s390-pci-inst.c b/hw/s390x/s390-pci-inst.c
> index e386d75d58..4c108fa8c4 100644
> --- a/hw/s390x/s390-pci-inst.c
> +++ b/hw/s390x/s390-pci-inst.c
> @@ -16,6 +16,7 @@
>  #include "exec/memory.h"
>  #include "qemu/error-report.h"
>  #include "system/hw_accel.h"
> +#include "hw/boards.h"
>  #include "hw/pci/pci_device.h"
>  #include "hw/s390x/s390-pci-inst.h"
>  #include "hw/s390x/s390-pci-bus.h"
> @@ -1008,17 +1009,25 @@ static int reg_ioat(CPUS390XState *env, 
> S390PCIBusDevice *pbdev, ZpciFib fib,
>      }
>  
>      /* currently we only support designation type 1 with translation */
> -    if (!(dt == ZPCI_IOTA_RTTO && t)) {
> +    if (t && !(dt == ZPCI_IOTA_RTTO)) {

What Thomas said ;-)

>          error_report("unsupported ioat dt %d t %d", dt, t);
>          s390_program_interrupt(env, PGM_OPERAND, ra);
>          return -EINVAL;
> +    } else if (!t && !pbdev->rtr_allowed) {
> +        error_report("relaxed translation not allowed");
> +        s390_program_interrupt(env, PGM_OPERAND, ra);
> +        return -EINVAL;
>      }
>  
>      iommu->pba = pba;
>      iommu->pal = pal;
>      iommu->g_iota = g_iota;
>  
> -    s390_pci_iommu_enable(iommu);
> +    if (t) {
> +        s390_pci_iommu_enable(iommu);
> +    } else {
> +        s390_pci_iommu_dm_enable(iommu);
> +    }
>  
>      return 0;
>  }
> diff --git a/hw/s390x/s390-pci-vfio.c b/hw/s390x/s390-pci-vfio.c
> index 7dbbc76823..dad525c81c 100644
> --- a/hw/s390x/s390-pci-vfio.c
> +++ b/hw/s390x/s390-pci-vfio.c
> @@ -133,11 +133,18 @@ static void s390_pci_read_base(S390PCIBusDevice *pbdev,
>  
>      /*
>       * If appropriate, reduce the size of the supported DMA aperture reported
> -     * to the guest based upon the vfio DMA limit.
> +     * to the guest based upon the vfio DMA limit.  This is applicable for
> +     * devices that are guaranteed to not use relaxed translation.  If the
> +     * device is capable of relaxed translation then we must advertise the
> +     * full aperture.  In this case, if translation is used then we will
> +     * rely on the vfio DMA limit counting and use RPCIT CC1 / status 16
> +     * to request the guest free DMA mappings when necessary.

Not a native speaker but I think there is a "to" missing in the last
sentence and I'd have used "as necessary".

>       */
> -    vfio_size = pbdev->iommu->max_dma_limit << TARGET_PAGE_BITS;
> -    if (vfio_size > 0 && vfio_size < cap->end_dma - cap->start_dma + 1) {
> -        pbdev->zpci_fn.edma = cap->start_dma + vfio_size - 1;
> +    if (!pbdev->rtr_allowed) {
> +        vfio_size = pbdev->iommu->max_dma_limit << TARGET_PAGE_BITS;
> +        if (vfio_size > 0 && vfio_size < cap->end_dma - cap->start_dma + 1) {
> +            pbdev->zpci_fn.edma = cap->start_dma + vfio_size - 1;
> +        }
>      }
>  }
>  
> diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
> index 3af613d4e9..c96cd0d4bb 100644
> --- a/hw/s390x/s390-virtio-ccw.c
> +++ b/hw/s390x/s390-virtio-ccw.c
> @@ -907,8 +907,13 @@ static void 
> ccw_machine_9_2_instance_options(MachineState *machine)
>  
>  static void ccw_machine_9_2_class_options(MachineClass *mc)
>  {
> +    static GlobalProperty compat[] = {
> +        { TYPE_S390_PCI_DEVICE, "relaxed-translation", "off", },
> +    };
> +
>      ccw_machine_10_0_class_options(mc);
>      compat_props_add(mc->compat_props, hw_compat_9_2, hw_compat_9_2_len);
> +    compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
>  }
>  DEFINE_CCW_MACHINE(9, 2);
>  
> diff --git a/include/hw/s390x/s390-pci-bus.h b/include/hw/s390x/s390-pci-bus.h
> index 2c43ea123f..27732247cf 100644
> --- a/include/hw/s390x/s390-pci-bus.h
> +++ b/include/hw/s390x/s390-pci-bus.h
> @@ -277,7 +277,9 @@ struct S390PCIIOMMU {
>      AddressSpace as;
>      MemoryRegion mr;
>      IOMMUMemoryRegion iommu_mr;
> +    MemoryRegion dm_mr;
>      bool enabled;
> +    bool direct_map;
>      uint64_t g_iota;
>      uint64_t pba;
>      uint64_t pal;
> @@ -362,6 +364,7 @@ struct S390PCIBusDevice {
>      bool interp;
>      bool forwarding_assist;
>      bool aif;
> +    bool rtr_allowed;

Nit: In the kernel in struct zpci_dev you used rtr_avail but "allowed"
in the comment, just for gerppability I'd prefer the names to match.

>      QTAILQ_ENTRY(S390PCIBusDevice) link;
>  };
>  
> @@ -389,6 +392,7 @@ int pci_chsc_sei_nt2_have_event(void);
>  void s390_pci_sclp_configure(SCCB *sccb);
>  void s390_pci_sclp_deconfigure(SCCB *sccb);
>  void s390_pci_iommu_enable(S390PCIIOMMU *iommu);
> +void s390_pci_iommu_dm_enable(S390PCIIOMMU *iommu);

Nit: I find "_dm_" a bit hard to map to "direct map". If you want two
letters I'd go for "_pt_" for "_iommu_pass_through_" or maybe
"_direct_map_".

>  void s390_pci_iommu_disable(S390PCIIOMMU *iommu);
>  void s390_pci_generate_error_event(uint16_t pec, uint32_t fh, uint32_t fid,
>                                     uint64_t faddr, uint32_t e);