iommu/arm-smmu-v3: permit users to disable msi polling

iommu/arm-smmu-v3: permit users to disable msi polling

Polling by MSI isn't necessarily faster than polling by SEV. Tests on
hi1620 show hns3 100G NIC network throughput can improve from 25G to
27G if we disable MSI polling while running 16 netperf threads sending
UDP packets in size 32KB. TX throughput can improve from 7G to 7.7G for
single thread.
The reason for the throughput improvement is that the latency to poll
the completion of CMD_SYNC becomes smaller. After sending a CMD_SYNC
in an empty cmd queue, typically we need to wait for 280ns using MSI
polling. But we only need around 190ns after disabling MSI polling.
This patch provides a command line option so that users can decide to
use MSI polling or not based on their tests.

Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20200827092957.22500-4-song.bao.hua@hisilicon.com
Signed-off-by: Will Deacon <will@kernel.org>