nvme: improve performance for virtual NVMe devices
This change provides a mechanism to reduce the number of MMIO doorbell writes for the NVMe driver. When running in a virtualized environment like QEMU, the cost of an MMIO is quite hefy here. The main idea for the patch is provide the device two memory location locations: 1) to store the doorbell values so they can be lookup without the doorbell MMIO write 2) to store an event index. I believe the doorbell value is obvious, the event index not so much. Similar to the virtio specification, the virtual device can tell the driver (guest OS) not to write MMIO unless you are writing past this value. FYI: doorbell values are written by the nvme driver (guest OS) and the event index is written by the virtual device (host OS). The patch implements a new admin command that will communicate where these two memory locations reside. If the command fails, the nvme driver will work as before without any optimizations. Contributions: Eric Northup <digitaleric@google.com> Frank Swiderski <fes@google.com> Ted Tso <tytso@mit.edu> Keith Busch <keith.busch@intel.com> Just to give an idea on the performance boost with the vendor extension: Running fio [1], a stock NVMe driver I get about 200K read IOPs with my vendor patch I get about 1000K read IOPs. This was running with a null device i.e. the backing device simply returned success on every read IO request. [1] Running on a 4 core machine: fio --time_based --name=benchmark --runtime=30 --filename=/dev/nvme0n1 --nrfiles=1 --ioengine=libaio --iodepth=32 --direct=1 --invalidate=1 --verify=0 --verify_fatal=0 --numjobs=4 --rw=randread --blocksize=4k --randrepeat=false Signed-off-by: Rob Nelson <rlnelson@google.com> [mlin: port for upstream] Signed-off-by: Ming Lin <mlin@kernel.org> [koike: updated for upstream] Signed-off-by: Helen Koike <helen.koike@collabora.co.uk> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Keith Busch <keith.busch@intel.com>
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