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From: Paolo Bonzini <pbonzini@redhat.com>
Date: Wed, 02 Nov 2011 20:50:54 +0100
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Subject: Re: [Qemu-devel] Do you have a use for a tester of virtio-scsi with
 CD drives ?
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To: qemu-devel@nongnu.org

On 11/02/2011 07:05 PM, Thomas Schmitt wrote:
>> >  The page length is indeed 18 for IDE and 20 for SCSI.  I made some changes
>> >  to that page recently, but left the 18 because I feared causing regression.
>> >  But if that is a bug, we can probably fix it in 1.0.
> This riddles me.
> In hw/scsi-disk.c:mode_sense_page() i see that the page 0x2A
> (now MODE_PAGE_CAPABILITIES) gets filled with 22 bytes, compliant
> to MMC-1. (Later MMCs have longer minimum sizes.)

Ok, in your counting I should have written 20 for IDE (0x12 + page 
number + page size) and 22 for SCSI (0x14 + page number + page size).

> But libburn receives only 20 of them, because the Page Length
> is reported as 0x12 in the 10th byte of the reply:
>    MODE SENSE
>    5a 00 2a 00 00 00 00 00 1c 00
>    From drive: 28b
>    00 22 70 00 00 00 00 00 2a 12 00 00 71 60 29 00 02 c2 00 02
>    02 00 02 c2 00 00 00 00
> Nevertheless i see in hw/scsi-disk.c
>    p[1] = 0x14;
> So how is this altered to 0x12 in the further course of processing ?

Because you're using an *IDE* (ATAPI) CD-ROM, not SCSI.  See hw/ide/atapi.c.

Paolo