From: Keith Busch <kbusch@kernel.org>
To: Vaibhav Nagarnaik <vnagarnaik@google.com>
Cc: Abbas Companywala <acompany@google.com>,
Bart Van Assche <bvanassche@google.com>,
linux-nvme@lists.infradead.org, Jens Axboe <axboe@fb.com>,
Eric Gouriou <egouriou@google.com>,
Christoph Hellwig <hch@lst.de>, "Mihai R." <dizzy@google.com>,
Sagi Grimberg <sagi@grimberg.me>
Subject: Re: NVMe PCI driver ignores SQHD from completion entries
Date: Sat, 5 Oct 2019 08:27:24 -0600 [thread overview]
Message-ID: <20191005142722.GA30437@keith-busch> (raw)
In-Reply-To: <CAL26m8+WdjrKvnXDZdP4d2ekznfORDUV2Td6U6-DY9gcOqoOUA@mail.gmail.com>
On Fri, Oct 04, 2019 at 11:27:30AM -0700, Vaibhav Nagarnaik wrote:
> According to NVMe spec:
> A Submission Queue entry has been consumed by the controller when a
> Completion Queue entry is posted that indicates that the Submission
> Queue Head Pointer has moved past the slot in which that Submission
> Queue entry was placed.
>
> Which means, the driver needs to verify SQ Head Pointer value reported
> in the completion entries before considering a particular SQ entry
> reusable. Otherwise it's undefined behavior.
The spec allows the controller to process and complete commands out of
order, but the controller must fetch those commands in order. It's in the
"Theory of Operation" section 1.4.
Checking SQ head is required only if the host might submit more commands
than there are entries. The Linux nvme driver allocates enough tags
for the depth of the queue, leaving one entry empty, so having a tag
available means the next sq entry must be available.
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next prev parent reply other threads:[~2019-10-05 14:27 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-04 18:27 NVMe PCI driver ignores SQHD from completion entries Vaibhav Nagarnaik
2019-10-05 14:27 ` Keith Busch [this message]
2019-10-08 0:32 ` Vaibhav Nagarnaik
2019-10-08 15:59 ` Keith Busch
2019-10-08 17:45 ` James Smart
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