Re: [PATCH 13/14] megaraid_sas: NVME passthru command support

[Douglas Gilbert <dgilbert@interlog.com>]

On 2018-01-09 11:45 AM, Christoph Hellwig wrote:
> On Tue, Jan 09, 2018 at 10:07:28PM +0530, Kashyap Desai wrote:
>> Chris -
>>
>> Overall NVME support behind MR controller is really a SCSI device. On top
>> of that, for MegaRaid, NVME device can be part of Virtual Disk and those
>> drive will not be exposed to the driver. User application may like to talk
>> to hidden NVME devices (part of VDs). This patch will extend the existing
>> interface for megaraid product in the same way it is currently supported
>> for other protocols like SMP, SATA pass-through.
>>
>> Example - Current smartmon is using megaraid.h (MFI headers) to send SATA
>> pass-through.
>>
>> https://github.com/mirror/smartmontools/blob/master/megaraid.h
> 
> And that is exactly the example of why we should have never allowed
> megaraid any private passthrough ioctls to start with.

Christoph,
Have you tried to do any serious work with <linux/nvme_ioctl.h> and
say compared it with FreeBSD and Microsoft's approach? No prize for
guessing which one is worst (and least extensible). Looks like the
Linux pass-through was at the end of a ToDo list and was "designed"
at 5 a.m in the morning.

RAID cards need a pass-through that allows them to address one of
many physical disks behind the virtual disk presented to OS.
Pass-throughs need to have uncommited room for extra parameters that
will be passed through as-is to the RAID LLD.

So until Christoph gives an example of how that can be done with
<linux/nvme_ioctl.h> then I would like to see Christoph's objection
ignored.


And as a maintainer of smartmontools, I would like to point out that
pretty well all supported RAIDs, on all platforms need specialized
pass-through code. Start by looking at os_linux.cpp and then at the
other OSes. And now smartmontools supports NVMe on most platforms
and at the pass-through level, it is just another one, and not a
particularly clean one.

IMO Intel had their chance on the pass-through front, and blew it.
It is now too late to fix it and that job (impossible ?) should not
fall to MegaRaid maintainers.

Douglas Gilbert