Re: [PATCH v2 2/3] s390x/pci: Honor DMA limits set by vfio

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 14 Sep 2020 18:29:29 -0400
Matthew Rosato <mjrosato@linux.ibm.com> wrote:

> When an s390 guest is using lazy unmapping, it can result in a very
> large number of oustanding DMA requests, far beyond the default
> limit configured for vfio.  Let's track DMA usage similar to vfio
> in the host, and trigger the guest to flush their DMA mappings
> before vfio runs out.
> 
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> ---
>  hw/s390x/s390-pci-bus.c  | 99 +++++++++++++++++++++++++++++++++++++++++++++---
>  hw/s390x/s390-pci-bus.h  |  9 +++++
>  hw/s390x/s390-pci-inst.c | 29 +++++++++++---
>  hw/s390x/s390-pci-inst.h |  3 ++
>  4 files changed, 129 insertions(+), 11 deletions(-)
> 

(...)

> @@ -737,6 +742,82 @@ static void s390_pci_iommu_free(S390pciState *s, PCIBus *bus, int32_t devfn)
>      object_unref(OBJECT(iommu));
>  }
>  
> +static bool s390_sync_dma_avail(int fd, unsigned int *avail)

Not sure I like the name. It sounds like the function checks whether
"sync dma" is available. Maybe s390_update_dma_avail()?

> +{
> +    struct vfio_iommu_type1_info *info;
> +    uint32_t argsz;
> +    bool rval = false;
> +    int ret;
> +
> +    if (avail == NULL) {
> +        return false;
> +    }
> +
> +    argsz = sizeof(struct vfio_iommu_type1_info);
> +    info = g_malloc0(argsz);
> +    info->argsz = argsz;
> +    /*
> +     * If the specified argsz is not large enough to contain all
> +     * capabilities it will be updated upon return.  In this case
> +     * use the updated value to get the entire capability chain.
> +     */
> +    ret = ioctl(fd, VFIO_IOMMU_GET_INFO, info);
> +    if (argsz != info->argsz) {
> +        argsz = info->argsz;
> +        info = g_realloc(info, argsz);
> +        info->argsz = argsz;
> +        ret = ioctl(fd, VFIO_IOMMU_GET_INFO, info);
> +    }
> +
> +    if (ret) {
> +        goto out;
> +    }
> +
> +    /* If the capability exists, update with the current value */
> +    rval = vfio_get_info_dma_avail(info, avail);

Adding vfio specific things into the generic s390 pci emulation code
looks a bit ugly... I'd prefer to factor that out into a separate file,
especially if you plan to add more vfio-specific things in the future.

> +
> +out:
> +    g_free(info);
> +    return rval;
> +}
> +

(...)

> diff --git a/hw/s390x/s390-pci-inst.c b/hw/s390x/s390-pci-inst.c
> index 2f7a7d7..6af9af4 100644
> --- a/hw/s390x/s390-pci-inst.c
> +++ b/hw/s390x/s390-pci-inst.c
> @@ -32,6 +32,9 @@
>          }                                                          \
>      } while (0)
>  
> +#define INC_DMA_AVAIL(iommu) if (iommu->dma_limit) iommu->dma_limit->avail++;
> +#define DEC_DMA_AVAIL(iommu) if (iommu->dma_limit) iommu->dma_limit->avail--;

Hm... maybe lowercase inline functions might be better here?

> +
>  static void s390_set_status_code(CPUS390XState *env,
>                                   uint8_t r, uint64_t status_code)
>  {

(...)

> @@ -620,6 +629,7 @@ int rpcit_service_call(S390CPU *cpu, uint8_t r1, uint8_t r2, uintptr_t ra)
>      S390PCIIOMMU *iommu;
>      S390IOTLBEntry entry;
>      hwaddr start, end;
> +    uint32_t dma_avail;
>  
>      if (env->psw.mask & PSW_MASK_PSTATE) {
>          s390_program_interrupt(env, PGM_PRIVILEGED, ra);
> @@ -675,8 +685,9 @@ int rpcit_service_call(S390CPU *cpu, uint8_t r1, uint8_t r2, uintptr_t ra)
>          }
>  
>          start += entry.len;
> -        while (entry.iova < start && entry.iova < end) {
> -            s390_pci_update_iotlb(iommu, &entry);
> +        dma_avail = 1; /* Assume non-zero dma_avail to start */
> +        while (entry.iova < start && entry.iova < end && dma_avail > 0) {
> +            dma_avail = s390_pci_update_iotlb(iommu, &entry);
>              entry.iova += PAGE_SIZE;
>              entry.translated_addr += PAGE_SIZE;
>          }
> @@ -689,7 +700,13 @@ err:
>          s390_pci_generate_error_event(error, pbdev->fh, pbdev->fid, start, 0);
>      } else {
>          pbdev->fmb.counter[ZPCI_FMB_CNT_RPCIT]++;
> -        setcc(cpu, ZPCI_PCI_LS_OK);
> +        if (dma_avail > 0) {

When I compile this (with a headers update), the compiler moans here
about an uninitialized variable.

> +            setcc(cpu, ZPCI_PCI_LS_OK);
> +        } else {
> +            /* vfio DMA mappings are exhausted, trigger a RPCIT */
> +            setcc(cpu, ZPCI_PCI_LS_ERR);
> +            s390_set_status_code(env, r1, ZPCI_RPCIT_ST_INSUFF_RES);
> +        }
>      }
>      return 0;
>  }

(...)