A question regarding "multiple SGL"

[Qiuxin (robert)]

Hi Christoph,

Thanks , got it.

Could you please do me favor to let me know the background why we ONLY support " MSDBD ==1"?   I am NOT trying to resist or oppose anything , I just want to know the reason.  You know,  it is a little wired for me, as  "MSDBD ==1" does not fulfill all the use cases which is depicted in the spec.

Best,
Robert Qiuxin
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-----邮件原件-----
发件人: Christoph Hellwig [mailto:hch at lst.de] 
发送时间: 2016年10月27日 14:41
收件人: 鑫愿
抄送: Bart Van Assche; Jens Axboe; linux-block at vger.kernel.org; James Bottomley; Martin K. Petersen; Mike Snitzer; linux-rdma at vger.kernel.org; Ming Lei; linux-nvme at lists.infradead.org; Keith Busch; Doug Ledford; linux-scsi at vger.kernel.org; Laurence Oberman; Christoph Hellwig; Tiger zhao; Qiuxin (robert)
主题: Re: A question regarding "multiple SGL"

Hi Robert,

There is no feature called "Multiple SGL in one NVMe capsule".  The NVMe over Fabrics specification allows a controller to advertise how many SGL descriptors it supports using the MSDBD Identify field:

"Maximum SGL Data Block Descriptors (MSDBD): This field indicates the maximum number of (Keyed) SGL Data Block descriptors that a host is allowed to place in a capsule. A value of 0h indicates no limit."

Setting this value to 1 is perfectly valid.  Similarly a host is free to chose any number of SGL descriptors between 0 (only for command that don't transfer data) to the limit imposed by the controller using the MSDBD field.

There are no plans to support a MSDBD value larger than 1 in the Linux NVMe target, and there are no plans to ever submit commands with multiple SGLs from the host driver either.

Cheers,
	Christoph