Re: [PATCH 3/3] s390x/pci: shrink DMA aperture to be bound by vfio DMA limit

[Eric Farman]

On Fri, 2022-10-28 at 15:47 -0400, Matthew Rosato wrote:
> Currently, s390x-pci performs accounting against the vfio DMA
> limit and triggers the guest to clean up mappings when the limit
> is reached. Let's go a step further and also limit the size of
> the supported DMA aperture reported to the guest based upon the
> initial vfio DMA limit reported for the container (if less than
> than the size reported by the firmware/host zPCI layer).  This
> avoids processing sections of the guest DMA table during global
> refresh that, for common use cases, will never be used anway, and
> makes exhausting the vfio DMA limit due to mismatch between guest
> aperture size and host limit far less likely and more indicitive
> of an error.
> 
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>

Reviewed-by: Eric Farman <farman@linux.ibm.com>

> ---
>  hw/s390x/s390-pci-vfio.c        | 11 +++++++++++
>  include/hw/s390x/s390-pci-bus.h |  1 +
>  2 files changed, 12 insertions(+)
> 
> diff --git a/hw/s390x/s390-pci-vfio.c b/hw/s390x/s390-pci-vfio.c
> index 2aefa508a0..99806e2a84 100644
> --- a/hw/s390x/s390-pci-vfio.c
> +++ b/hw/s390x/s390-pci-vfio.c
> @@ -84,6 +84,7 @@ S390PCIDMACount
> *s390_pci_start_dma_count(S390pciState *s,
>      cnt->users = 1;
>      cnt->avail = avail;
>      QTAILQ_INSERT_TAIL(&s->zpci_dma_limit, cnt, link);
> +    pbdev->iommu->max_dma_limit = avail;
>      return cnt;
>  }
>  
> @@ -103,6 +104,7 @@ static void s390_pci_read_base(S390PCIBusDevice
> *pbdev,
>      struct vfio_info_cap_header *hdr;
>      struct vfio_device_info_cap_zpci_base *cap;
>      VFIOPCIDevice *vpci =  container_of(pbdev->pdev, VFIOPCIDevice,
> pdev);
> +    uint64_t vfio_size;
>  
>      hdr = vfio_get_device_info_cap(info,
> VFIO_DEVICE_INFO_CAP_ZPCI_BASE);
>  
> @@ -122,6 +124,15 @@ static void s390_pci_read_base(S390PCIBusDevice
> *pbdev,
>      /* The following values remain 0 until we support other FMB
> formats */
>      pbdev->zpci_fn.fmbl = 0;
>      pbdev->zpci_fn.pft = 0;
> +
> +    /*
> +     * If appropriate, reduce the size of the supported DMA aperture
> reported
> +     * to the guest based upon the vfio DMA limit.
> +     */
> +    vfio_size = pbdev->iommu->max_dma_limit << TARGET_PAGE_BITS;
> +    if (vfio_size < (cap->end_dma - cap->start_dma + 1)) {
> +        pbdev->zpci_fn.edma = cap->start_dma + vfio_size - 1;
> +    }
>  }
>  
>  static bool get_host_fh(S390PCIBusDevice *pbdev, struct
> vfio_device_info *info,
> diff --git a/include/hw/s390x/s390-pci-bus.h b/include/hw/s390x/s390-
> pci-bus.h
> index 0605fcea24..1c46e3a269 100644
> --- a/include/hw/s390x/s390-pci-bus.h
> +++ b/include/hw/s390x/s390-pci-bus.h
> @@ -278,6 +278,7 @@ struct S390PCIIOMMU {
>      uint64_t g_iota;
>      uint64_t pba;
>      uint64_t pal;
> +    uint64_t max_dma_limit;
>      GHashTable *iotlb;
>      S390PCIDMACount *dma_limit;
>  };