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

[Sagi Grimberg]

On 18/10/2024 8:14, Christoph Hellwig wrote:
> 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.

I think the ctrl misc device attributes are fine to expose this and other
types of attributes (like we already do today).