Re: [PATCH 2/3] s390x/pci: coalesce unmap operations

[Eric Farman]

On Fri, 2022-10-28 at 15:47 -0400, Matthew Rosato wrote:
> Currently, each unmapped page is handled as an individual iommu
> region notification.  Attempt to group contiguous unmap operations
> into fewer notifications to reduce overhead.
> 
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> ---
>  hw/s390x/s390-pci-inst.c | 51
> ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 51 insertions(+)
> 
> diff --git a/hw/s390x/s390-pci-inst.c b/hw/s390x/s390-pci-inst.c
> index 7cc4bcf850..66e764f901 100644
> --- a/hw/s390x/s390-pci-inst.c
> +++ b/hw/s390x/s390-pci-inst.c
> @@ -640,6 +640,8 @@ static uint32_t
> s390_pci_update_iotlb(S390PCIIOMMU *iommu,
>          }
>          g_hash_table_remove(iommu->iotlb, &entry->iova);
>          inc_dma_avail(iommu);
> +        /* Don't notify the iommu yet, maybe we can bundle
> contiguous unmaps */
> +        goto out;
>      } else {
>          if (cache) {
>              if (cache->perm == entry->perm &&
> @@ -663,15 +665,44 @@ static uint32_t
> s390_pci_update_iotlb(S390PCIIOMMU *iommu,
>          dec_dma_avail(iommu);
>      }
>  
> +    /*
> +     * All associated iotlb entries have already been cleared,
> trigger the
> +     * unmaps.
> +     */
>      memory_region_notify_iommu(&iommu->iommu_mr, 0, event);
>  
>  out:
>      return iommu->dma_limit ? iommu->dma_limit->avail : 1;
>  }
>  
> +static void s390_pci_batch_unmap(S390PCIIOMMU *iommu, uint64_t iova,
> +                                 uint64_t len)
> +{
> +    uint64_t remain = len, start = iova, end = start + len - 1,
> mask, size;
> +    IOMMUTLBEvent event = {
> +        .type = IOMMU_NOTIFIER_UNMAP,
> +        .entry = {
> +            .target_as = &address_space_memory,
> +            .translated_addr = 0,
> +            .perm = IOMMU_NONE,
> +        },
> +    };
> +
> +    while (remain >= TARGET_PAGE_SIZE) {
> +        mask = dma_aligned_pow2_mask(start, end, 64);
> +        size = mask + 1;
> +        event.entry.iova = start;
> +        event.entry.addr_mask = mask;
> +        memory_region_notify_iommu(&iommu->iommu_mr, 0, event);
> +        start += size;
> +        remain -= size;
> +    }
> +}
> +
>  int rpcit_service_call(S390CPU *cpu, uint8_t r1, uint8_t r2,
> uintptr_t ra)
>  {
>      CPUS390XState *env = &cpu->env;
> +    uint64_t iova, coalesce = 0;
>      uint32_t fh;
>      uint16_t error = 0;
>      S390PCIBusDevice *pbdev;
> @@ -742,6 +773,21 @@ int rpcit_service_call(S390CPU *cpu, uint8_t r1,
> uint8_t r2, uintptr_t ra)
>              break;
>          }
>  
> +        /*
> +         * If this is an unmap of a PTE, let's try to coalesce
> multiple unmaps
> +         * into as few notifier events as possible.
> +         */
> +        if (entry.perm == IOMMU_NONE && entry.len ==
> TARGET_PAGE_SIZE) {
> +            if (coalesce == 0) {
> +                iova = entry.iova;
> +            }
> +            coalesce += entry.len;
> +        } else if (coalesce > 0) {
> +            /* Unleash the coalesced unmap before processing a new
> map */
> +            s390_pci_batch_unmap(iommu, iova, coalesce);
> +            coalesce = 0;
> +        }
> +
>          start += entry.len;
>          while (entry.iova < start && entry.iova < end) {
>              if (dma_avail > 0 || entry.perm == IOMMU_NONE) {
> @@ -759,6 +805,11 @@ int rpcit_service_call(S390CPU *cpu, uint8_t r1,
> uint8_t r2, uintptr_t ra)
>              }
>          }
>      }
> +    if (coalesce) {

I'd guess this should be "coalesce > 0" as above. Regardless,

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

> +            /* Unleash the coalesced unmap before finishing rpcit */
> +            s390_pci_batch_unmap(iommu, iova, coalesce);
> +            coalesce = 0;
> +    }
>      if (again && dma_avail > 0)
>          goto retry;
>  err: