Re: [PATCH 1/3] s390x/pci: RPCIT second pass when mappings exhausted

[Eric Farman]

On Fri, 2022-10-28 at 15:47 -0400, Matthew Rosato wrote:
> If we encounter a new mapping while the number of available DMA
> entries
> in vfio is 0, we are currently skipping that mapping which is a
> problem
> if we manage to free up DMA space after that within the same RPCIT --
> we will return to the guest with CC0 and have not mapped everything
> within the specified range.  This issue was uncovered while testing
> changes to the s390 linux kernel iommu/dma code, where a different
> usage pattern was employed (new mappings start at the end of the
> aperture and work back towards the front, making us far more likely
> to encounter new mappings before invalidated mappings during a
> global refresh).
> 
> Fix this by tracking whether any mappings were skipped due to vfio
> DMA limit hitting 0; when this occurs, we still continue the range
> and unmap/map anything we can - then we must re-run the range again
> to pickup anything that was missed.  This must occur in a loop until
> all requests are satisfied (success) or we detect that we are still
> unable to complete all mappings (return ZPCI_RPCIT_ST_INSUFF_RES).
> 
> Link:
> https://lore.kernel.org/linux-s390/20221019144435.369902-1-schnelle@linux.ibm.com/
> Fixes: 37fa32de70 ("s390x/pci: Honor DMA limits set by vfio")
> Reported-by: Niklas Schnelle <schnelle@linux.ibm.com>
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>

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

> ---
>  hw/s390x/s390-pci-inst.c | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/s390x/s390-pci-inst.c b/hw/s390x/s390-pci-inst.c
> index 20a9bcc7af..7cc4bcf850 100644
> --- a/hw/s390x/s390-pci-inst.c
> +++ b/hw/s390x/s390-pci-inst.c
> @@ -677,8 +677,9 @@ int rpcit_service_call(S390CPU *cpu, uint8_t r1,
> uint8_t r2, uintptr_t ra)
>      S390PCIBusDevice *pbdev;
>      S390PCIIOMMU *iommu;
>      S390IOTLBEntry entry;
> -    hwaddr start, end;
> +    hwaddr start, end, sstart;
>      uint32_t dma_avail;
> +    bool again;
>  
>      if (env->psw.mask & PSW_MASK_PSTATE) {
>          s390_program_interrupt(env, PGM_PRIVILEGED, ra);
> @@ -691,7 +692,7 @@ int rpcit_service_call(S390CPU *cpu, uint8_t r1,
> uint8_t r2, uintptr_t ra)
>      }
>  
>      fh = env->regs[r1] >> 32;
> -    start = env->regs[r2];
> +    sstart = start = env->regs[r2];
>      end = start + env->regs[r2 + 1];
>  
>      pbdev = s390_pci_find_dev_by_fh(s390_get_phb(), fh);
> @@ -732,6 +733,9 @@ int rpcit_service_call(S390CPU *cpu, uint8_t r1,
> uint8_t r2, uintptr_t ra)
>          goto err;
>      }
>  
> + retry:
> +    start = sstart;
> +    again = false;
>      while (start < end) {
>          error = s390_guest_io_table_walk(iommu->g_iota, start,
> &entry);
>          if (error) {
> @@ -739,13 +743,24 @@ 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 &&
> -               (dma_avail > 0 || entry.perm == IOMMU_NONE)) {
> -            dma_avail = s390_pci_update_iotlb(iommu, &entry);
> -            entry.iova += TARGET_PAGE_SIZE;
> -            entry.translated_addr += TARGET_PAGE_SIZE;
> +        while (entry.iova < start && entry.iova < end) {
> +            if (dma_avail > 0 || entry.perm == IOMMU_NONE) {
> +                dma_avail = s390_pci_update_iotlb(iommu, &entry);
> +                entry.iova += TARGET_PAGE_SIZE;
> +                entry.translated_addr += TARGET_PAGE_SIZE;
> +            } else {
> +                /*
> +                 * We are unable to make a new mapping at this time,
> continue
> +                 * on and hopefully free up more space.  Then
> attempt another
> +                 * pass.
> +                 */
> +                again = true;
> +                break;
> +            }
>          }
>      }
> +    if (again && dma_avail > 0)
> +        goto retry;
>  err:
>      if (error) {
>          pbdev->state = ZPCI_FS_ERROR;