Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive

[Hannes Reinecke <hare@suse.de>]

On 08/24/2012 09:56 AM, Paolo Bonzini wrote:
> Il 24/08/2012 02:45, Nicholas A. Bellinger ha scritto:
>> So up until very recently, TCM would accept an I/O request for an DATA
>> I/O type CDB with a max_sectors larger than the reported max_sectors for
>> it's TCM backend (regardless of backend type), and silently generate N
>> backend 'tasks' to complete the single initiator generated command.
>
> This is what QEMU does if you use scsi-block, except for MMC devices
> (because of the insanity of the commands used for burning).
>
>> Also FYI for Paolo, for control type CDBs I've never actually seen an
>> allocation length exceed max_sectors, so in practice AFAIK this only
>> happens for DATA I/O type CDBs.
>
> Yes, that was my impression as well.
>
>> This was historically required by the pSCSI backend driver (using a
>> number of old SCSI passthrough interfaces) in order to support this very
>> type of case described above, but over the years the logic ended up
>> creeping into various other non-passthrough backend drivers like IBLOCK
>> +FILEIO.  So for v3.6-rc1 code, hch ended up removing the 'task' logic
>> thus allowing backends (and the layers below) to the I/O sectors >
>> max_sectors handling work, allowing modern pSCSI using struct request to
>> do the same.  (hch assured me this works now for pSCSI)
>
> So now LIO and QEMU work the same.  (Did he test tapes too?)
>
>> Anyways, I think having the guest limit virtio-scsi DATA I/O to
>> max_sectors based upon the host accessible block limits is reasonable
>> approach to consider.  Reducing this value even further based upon the
>> lowest max_sectors available amongst possible migration hosts would be a
>> good idea here to avoid having to reject any I/O's exceeding a new
>> host's device block queue limits.
>
> Yeah, it's reasonable _assuming it is needed at all_.  For disks, it is
> not needed.  For CD-ROMs it is, but right now we have only one report
> and it is using USB so we don't know if the problem is in the drive or
> rather in the USB bridge (whose quality usually leaves much to be desired).
>
> So in the only observed case, the fix would really be a workaround; the
> right thing to do with USB devices is to use USB passthrough.
>

Hehe. So finally someone else stumbled across this one.

All is fine and dandy as long as you're able to use scsi-disk.
As soon as you're forced to use scsi-generic we're in trouble.

With scsi-generic we actually have two problems:
1) scsi-generic just acts as a pass-through and passes the commands
    as-is, including the scatter-gather information as formatted by
    the guest. So the guest could easily format an SG_IO comand
    which will not be compatible with the host.
2) The host is not able to differentiate between a malformed
    SG_IO command and a real I/O error; in both cases it'll return
    -EIO.

So we can fix this by either
a) ignore (as we do nowadays :-)
b) Fixup scsi-generic to inspect and modify SG_IO information
    to ensure the host-limits are respected
c) Fixup the host to differentiate between a malformed SG_IO
    and a real I/O error.

c) would only be feasible for Linux et al. _personally_ I would prefer 
that approach, as I fail to see why we cannot return a proper error code 
here.
But I already can hear the outraged cry 'POSIX! POSIX!', so I guess it's 
not going to happen anytime soon.
So I would vote for b).
Yes, it's painful. But in the long run we'll have to do an SG_IO 
inspection anyway, otherwise we'll always be susceptible to malicious 
SG_IO attacks.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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