Re: [PATCH v4 1/2] s390x/pci: add support for guests that request direct mapping

[David Hildenbrand <david@redhat.com>]

On 07.02.25 21:56, 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         | 38 +++++++++++++++++++++++++++++++--
>   hw/s390x/s390-pci-inst.c        | 13 +++++++++--
>   hw/s390x/s390-pci-vfio.c        | 23 ++++++++++++++++----
>   hw/s390x/s390-virtio-ccw.c      |  5 +++++
>   include/hw/s390x/s390-pci-bus.h |  4 ++++
>   5 files changed, 75 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
> index eead269cc2..81e5843c81 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 +722,45 @@ 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_direct_map_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.
> +     */
> +    g_autofree char *name = g_strdup_printf("iommu-dm-s390-%04x",
> +                                            iommu->pbdev->uid);

Empty line.

> +    memory_region_init_alias(&iommu->dm_mr, OBJECT(&iommu->mr), name,
> +                             get_system_memory(), 0,
> +                             s390_get_memory_limit(s390ms));
> +    iommu->enabled = true;
 > +    iommu->direct_map = true;


You could dynamically allocate the dm_mr instead, and use that as 
indication if the direct mapping is active. Whatever you prefer.


Nothing else jumped at me, thanks!

-- 
Cheers,

David / dhildenb