Since there is discussion in the Linux NVMe Driver community to add NVMe Atomic Write
support, it would be desirable to test it with qemu nvme emulation.
 
Initially, this RFC will focus on supporting NVMe controller atomic write parameters(AWUN,
AWUPF, and ACWU) but will be extended to support Namespace parameters (NAWUN, NAWUPF
and NACWU).
 
Atomic Write Parameters for NVMe QEMU
-------------------------------------
New NVMe QEMU Parameters (See NVMe Specification for details):
        atomic.dn (default off) - Set the value of Disable Normal.
        atomic.awun=UINT16 (default: 0)
        atomic.awupf=UINT16 (default: 0)
        atomic.acwu=UINT16 (default: 0)
 
qemu command line example:
        qemu-system-x86_64 -cpu host --enable-kvm -smp cpus=4 -no-reboot -m 8192M -drive file=./disk.img,if=ide \
        -boot c -device e1000,netdev=net0,mac=DE:CC:CC:EF:99:88 -netdev tap,id=net0 \
        -device nvme,id=nvme-ctrl-0,serial=nvme-1,atomic.dn=off,atomic.awun=63,atomic.awupf=63,atomic.acwu=0 \
        -drive file=./nvme.img,if=none,id=nvm-1 -device nvme-ns,drive=nvm-1,bus=nvme-ctrl-0 nvme-ns,drive=nvm-1,bus=nvme-ctrl-0
 
Making Writes Atomic:
---------------------
- Prior to a command being pulled off the SQ and executed, a check is made to see if it
  conflicts "atomically" with a currently executing command.
- All currently executing commands on the same namespace, across all SQs need to be checked.
- If an atomic conflict is detected, the command is not started and remains on the queue.
 
Testing
-------
NVMe QEMU Parameters used: atomic.dn=off,atomic.awun=63,atomic.awupf=63,atomic.acwu=0
 
# nvme id-ctrl /dev/nvme0 | grep awun
awun      : 63
# nvme id-ctrl /dev/nvme0 | grep awupf
awupf     : 63
# nvme id-ctrl /dev/nvme0 | grep acwu
acwu      : 0    < Since qemu-nvme doesn't support Compare and Write, this is always zero
# nvme get-feature /dev/nvme0  -f 0xa
get-feature:0x0a (Write Atomicity Normal), Current value:00000000
#
 
# fio --filename=/dev/nvme0n1 --direct=1 --rw=randwrite --bs=32k --iodepth=256 --name=iops --numjobs=50 --verify=crc64 --verify_fatal=1 --ioengine=libaio
 
When executed without atomic write support, eventually the following error will be
observed:
 
        crc64: verify failed at file /dev/nvme0n1 offset 857669632, length 32768
(requested block: offset=857669632, length=32768, flags=88)
            Expected CRC: 9c87d3539dafdca0
            Received CRC: d521f7ea3b69d2ee
 
When executed with atomic write support, this error no longer happens.
 
Questions
---------
AWUN vs AWUPF - Does the nvme emulation need to do treat these differently? Currently the
larger of the two will be used as the max atomic write size.
 
Future Work
-----------
- Namespace support (NAWUN, NAWUPF and NACWU)
- Namespace Boundary support (NABSN, NABO, and NABSPF)
- Atomic Compare and Write Unit (ACWU)

Alan Adamson (1):
  nvme: add atomic write support

 hw/nvme/ctrl.c | 147 ++++++++++++++++++++++++++++++++++++++++++++++++-
 hw/nvme/nvme.h |  17 ++++++
 2 files changed, 163 insertions(+), 1 deletion(-)

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2.39.3