Re: [PATCH] nvme-sysfs: display max_hw_sectors_kb without requiring namespaces

[Christoph Hellwig <hch@lst.de>]

On Thu, Oct 17, 2024 at 10:40:36AM -0600, Keith Busch wrote:
> On Wed, Oct 16, 2024 at 09:31:08PM +0000, Abhishek Bapat wrote:
> > max_hw_sectors based on DMA optimized limitation") introduced a
> > limitation on the value of max_hw_sectors_kb, restricting it to 128KiB
> > (MDTS = 5). This restricion was implemented to mitigate lockups
> > encountered in high-core count AMD servers.
> 
> There are other limits that can constrain transfer sizes below the
> device's MDTS. For example, the driver can only preallocate so much
> space for DMA and SGL descriptors, so 8MB is the current max transfer
> sizes the driver can support, and a device's MDTS can be much bigger
> than that.

Yes.  Plus the virt boundary for PRPs, and for non-PCIe tranfers
there's also plenty of other hardware limits due to e.g. the FC HBA
and the RDMA HCA limit.  There's also been some talk of a new PCIe
SGL variant with hard limits.

So I agree that exposting limits on I/O would be very useful, but it's
also kinda non-trivial.

> Anyway, yeah, I guess having a controller generic way to export this
> sounds like a good idea, but I wonder if the nvme driver is the right
> place to do it. The request_queue has all the limits you need to know
> about, but these are only exported if a gendisk is attached to it.
> Maybe we can create a queue subdirectory to the char dev too. 

If we want it controller wide to e.g. include the admin queue the
gendisk won't really help unfortunately.