On 4/8/25 11:47 PM, Klaus Jensen wrote:
On Apr 8 15:56, Alan Adamson wrote:
While testing Linux atomic writes with qemu-nvme v10.0.0-rc1, Linux was
incorrectly displaying atomic_write_max_bytes
# cat /sys/block/nvme0n1/queue/atomic_write_max_bytes
0
# nvme id-ctrl /dev/nvme0n1 | grep awupf
awupf : 15
#
Since AWUPF was set to 15, it was expected atomic_write_max_bytes would
be set to 8192.
The commit cd59f50ab017 ("hw/nvme: always initialize a subsystem")
introduced this behavior. The commit hardcodes the subsystem cmic bit
to ON which caused the Linux NVMe driver to treat the namespace as
multi-pathed which uncovered a bug with how Atomic Write Queue Limits
were being inherited. This Linux issue is being addressed, but the
question was asked of why the subsystem cmic bit was hardcoded to ON.
Most NVMe devices today don't set cmic to ON. Shouldn't the setting of
this bit be a settable parameter?
<subsystem>,cmic=BOOLEAN (default: off)
Example:
-device nvme-subsys,id=subsys0,cmic=on \
-device
nvme,serial=deadbeef,id=nvme0,subsys=subsys0,atomic.dn=off,atomic.awun=31,atomic.awupf=15
\
-drive id=ns1,file=/dev/nullb3,if=none \
-device nvme-ns,drive=ns1,bus=nvme0,nsid=1,shared=false
Alan Adamson (1):
hw/nvme: create parameter to enable/disable cmic on subsystem
hw/nvme/ctrl.c | 5 ++++-
hw/nvme/nvme.h | 1 +
hw/nvme/subsys.c | 1 +
3 files changed, 6 insertions(+), 1 deletion(-)
--
2.43.5
Hi Alan,
I agree that it would be better for CMIC.MCTRS to remain zero for
single-controller subsystems, but I'd rather not add the parameter
without verifying it (it would be invalid to have a multi-controller
subsystem with CMIC.MCTRS zeroed).
An improvement would be to just set it if the subsystems contains
multiple controllers.
Prior to commit cd59f50ab017 we would also statically set CMIC.MCTRS to
1 if a subsystem was configured, regardless of the number of
controllers.
Klaus,
I'll send out a v2 that automatically sets CMIC.MCTRS for each
controller of a multi-controller subsystem. For single controller
subsystems, CMIC.MCTRS will default to off and the cmic parameter can be
used to change CMIC.MCTRS to on.
Thanks!
Alan
