[EXT] Re: nvme may get timeout from dd when using different non-prefetch mmio outbound/ranges

[Keith Busch]

On Tue, Oct 26, 2021 at 03:40:54AM +0000, Li Chen wrote:
> My nvme is " 05:00.0 Non-Volatile memory controller: Samsung Electronics Co Ltd NVMe SSD Controller 980". From its datasheet, https://s3.ap-northeast-2.amazonaws.com/global.semi.static/Samsung_NVMe_SSD_980_Data_Sheet_Rev.1.1.pdf, it says nothing about CMB/SQEs, so I'm not sure. Is there other ways/tools(like nvme-cli) to query?

The driver will export a sysfs property for it if it is supported:

  # cat /sys/class/nvme/nvme0/cmb

If the file doesn't exist, then /dev/nvme0 doesn't have the capability.

> > > I don't know how to interpret "ranges".  Can you supply the dmesg and
> > > "lspci -vvs 0000:05:00.0" output both ways, e.g.,
> > >
> > >   pci_bus 0000:00: root bus resource [mem 0x7f800000-0xefffffff window]
> > >   pci_bus 0000:00: root bus resource [mem 0xfd000000-0xfe7fffff window]
> > >   pci 0000:05:00.0: [vvvv:dddd] type 00 class 0x...
> > >   pci 0000:05:00.0: reg 0x10: [mem 0x.....000-0x.....fff ...]
> > >
> > > > Question:
> > > > 1.  Why dd can cause nvme timeout? Is there more debug ways?
> > 
> > That means the nvme controller didn't provide a response to a posted
> > command within the driver's latency tolerance.
> 
> FYI, with the help of pci bridger's vendor, they find something interesting: "From catc log, I saw some memory read pkts sent from SSD card, but its memory range is within the memory range of switch down port. So, switch down port will replay UR pkt. It seems not normal." and "Why SSD card send out some memory pkts which memory address is within switch down port's memory range. If so, switch will response UR pkts". I also don't understand how can this happen?

I think we can safely assume you're not attempting peer-to-peer, so that
behavior as described shouldn't be happening. It sounds like the memory
windows may be incorrect. The dmesg may help to show if something appears
wrong.