linux-kernel - Re: [PATCH] nvme-pci: fix prp list allocation

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Message-ID: <20220212210258.GA1660900@dhcp-10-100-145-180.wdc.com>
Date:   Sat, 12 Feb 2022 13:02:58 -0800
From:   Keith Busch <kbusch@...nel.org>
To:     Matt Waltz <matthewwaltzis@...il.com>
Cc:     Jens Axboe <axboe@...com>, Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme-pci: fix prp list allocation

On Sat, Feb 12, 2022 at 01:53:59PM -0700, Matt Waltz wrote:
> That is the wrong part of the specification, describing the PRP entries,
> not the list pointer. 

The text for qword alignment rules specifically says "PRP List entry",
as is associated with command's prp2 field.

> The relevant portion of the spec (2.0a figure
> 109 page 129) says:
> 
>     If this entry is not the first PRP entry in the command or a
>     PRP List pointer in a command, then the Offset portion of this
>     field shall be cleared to 0h

That only applies to chaining list elements. It does not apply to the
"PRP list pointer in a command". The driver allocates from the 256B dma
pool only for the command's PRP list pointer, so we're fine.