linux-kernel - Re: [PATCH] nvme-sysfs: display max_hw_sectors

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Message-ID: <6905cfbc-2ae2-487f-aebf-e4b944f2dda4@grimberg.me>
Date: Mon, 21 Oct 2024 00:25:21 +0300
From: Sagi Grimberg <sagi@...mberg.me>
To: Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>
Cc: Abhishek Bapat <abhishekbapat@...gle.com>, Jens Axboe <axboe@...nel.dk>,
 Prashant Malani <pmalani@...gle.com>, linux-nvme@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme-sysfs: display max_hw_sectors_kb without requiring
 namespaces




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).