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

[Keith Busch <kbusch@kernel.org>]

On Thu, Oct 17, 2024 at 02:32:18PM -0700, Abhishek Bapat wrote:
> On Thu, Oct 17, 2024 at 9:40 AM Keith Busch <kbusch@kernel.org> 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.
> >
> > 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.
> 
> Are you suggesting that all the files from the queue subdirectory should
> be included in the char dev (/sys/class/nvme/nvmeX/queue/)? Or that
> just the max_hw_sectors_kb value should be shared within the queue
> subdirectory? And if not the nvme driver, where else can this be done
> from?

You'd may want to know max_sectors_kb, dma_alignment, nr_requests,
virt_boundary_mask. Maybe some others.

The request_queue is owned by the block layer, so that seems like an
okay place to export it, but attached to some other device's sysfs
directory instead of a gendisk.

I'm just suggesting this because it doesn't sound like this is an nvme
specific problem.