megaraid_sas Fails to Enumerate VDs When mem_encrypt=on in DELL EMC AMD Based Servers

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Environment

SUSE Linux Enterprise Server 15 Service Pack 1 (SLES 15 SP1)

Situation

This issue affects DELL EMC AMD based Servers with a H330 (or any PERC9) storage controller with VDs created underneath it. When booting to SLES 15 SP1 GM installer iso with mem_encrypt=on kernel boot parameter, no VDs are listed by the installer. OS logs indicate that the megaraid_sas driver has failed to allocate DMA buffers hence no VDs are exposed.

Resolution

- Workaround: Set kernel boot parameters "iommu=nopt iommu.passthrough=0" to disable iommu passthrough. These parameters are required as a work around for the old kernel.

- Permanent fix: Update kernel to kernel-default-4.12.14-197.18.1 or later. The kernel fix is accepted upstream and details can be found at https://lkml.org/lkml/2019/8/19/459.

Cause

SME with passthrough mode is fragile with devices requiring SWIOTLB, mainly because SWIOTLB has a maximum allocation size of 256kb and a limit overall size of the bounce buffer.

Therefore having IOMMU in translation mode by default is better when SME is active on a system.

Hence now as a resolution it disables IOMMU Passthrough mode when SME is active even when CONFIG_IOMMU_DEFAULT_PASSTHROUGH=y is set.

Disclaimer

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