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Virtual machine network connectivity is lost after hot adding RAM

Last Updated: Sep 08, 2016 04:30PM UTC

Symptoms

After hot adding memory to a Linux-based virtual machine, network connectivity is lost. The following messages may be observed in the virtual machine console:
ACPI:memory_hp:Memory online failed for 0x148000000 - 0x1c8000000
nommu_map_single: overflow 147bf3000+21 of device mask ffffffff e1000 0000:00:05.0: TX DMA map failed
nommu_map_single: overflow 147bf3000+21 of device mask ffffffff e1000 0000:00:05.0: TX DMA map failed
nommu_map_single: overflow 147bf3000+21 of device mask ffffffff e1000 0000:00:05.0: TX DMA map failed
nommu_map_single: overflow 147bf3000+21 of device mask ffffffff e1000 0000:00:05.0: TX DMA map failed


These symptoms are only observed if the virtual machine was started with RAM of less than 4GB and after hot adding memory, the total RAM amount is higher than 4GB.

Cause

By default, 64-bit Linux distributions without IOMMU and with RAM >4GB use bounce buffers (swiotlb) for DMA for 32-bit PCI device drivers. If RAM <4GB, then bounce buffers will not be used for such drivers to reduce memory overhead.

When a virtual machine starts with less than 4GB of RAM, bounce buffers are not enabled; hence, after adding memory >4GB, PCI drivers may fail to work properly.

Resolution

There are several general recommendations you can take to avoid the issue:

1. If the virtual machine starts with a RAM amount of less than 4GB:
     a. While hot adding RAM, do not cross the 4GB limit for the total RAM amount.
     b. Append iommu=soft to the kernel line in the GRUB configuration to be able to go beyond the 4GB RAM limit.
2. If the virtual machine starts with a RAM amount greater than 4GB, the issue will not be observed.

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