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.

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