linux-kernel - Re: [PATCH v3 1/2] virtio-scsi: first version

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Date:	Mon, 19 Dec 2011 10:00:34 -0500
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	Hannes Reinecke <hare@...e.de>, linux-kernel@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Stefan Hajnoczi <stefanha@...ux.vnet.ibm.com>,
	Mike Christie <michaelc@...wisc.edu>
Subject: Re: [PATCH v3 1/2] virtio-scsi: first version

On 11-12-19 08:11 AM, Paolo Bonzini wrote:
> On 12/19/2011 01:22 PM, Hannes Reinecke wrote:
>>> + case VIRTIO_SCSI_S_UNDERRUN:
>>> + set_host_byte(sc, DID_ERROR);
>>> + break;
>> Hmm. Not sure this is correct.
>> UNDERRUN could be a valid state, eg it is quite common to send
>> an INQUIRY with a length of 255 bytes. And only evaluate the
>> bytes which are actually send back.
>
> This happens when you send an INQUIRY with a sglist of 255 bytes and an
> ALLOCATION LENGTH of 300 bytes. The spec says "VIRTIO_SCSI_S_UNDERRUN [is
> returned] if the content of the CDB requires transferring more data than is
> available in the data buffers".

Overrun! That is overrun.

Underrun conventionally refers to the data-in buffer
containing less data than the transport has allowed
for. It is detected at the completion of a SCSI command
and is used to report to the application client that
bytes beyond a certain position in the data-in buffer
are not valid (i.e. ignore them).

A mismatch between the allocation length field inside a
SCSI command (a difficult and dangerous area to snoop in)
and the data-in buffer length indicated by sglist should be
called something else. Arguably it should not be treated
as an error until an overrun occurs.

I think the 'case VIRTIO_SCSI_S_UNDERRUN:' line deserves a
comment due to its misleading use of the word "underrun".
It is reporting a potential or actual _overrun_.

Doug Gilbert

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