On Thu, 2012-08-23 at 11:08 +0100, Stefan Hajnoczi wrote: > On Thu, Aug 23, 2012 at 11:03 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > Il 23/08/2012 11:31, Cong Meng ha scritto: > >>> For disks, this should be fixed simply by using scsi-block instead of > >>> scsi-generic. > >>> > >>> CD-ROMs are indeed more complicated because burning CDs cannot be done > >>> with syscalls. :/ > >> > >> So, as the problem exist to CD-ROM, I will continue to get these patches > >> move on. > > > > I'm still trying to understand the extent of the problem. > > > > The problem occurs for _USB_ CD-ROMs according to Ben. Passthrough of > > USB storage devices should be done via USB passthrough, not virtio-scsi. > > If we do USB passthrough via the SCSI layer we miss on all the quirks > > that the OS may do based on the USB product/vendor pairs. There's no > > end to these, and some of the quirks may cause the device to lock up or > > corruption. > > > > I'd rather see a reproducer using SAS/ATA/ATAPI disks before punting. > > This issue affects passthrough: either an entire sg device or at least > a SG_IO ioctl (e.g. a non-READ/WRITE SCSI command). > > To reproduce it, check host queue limits and guest virtio-scsi queue > limits. Then pick a command that can exceed the limits and try it > from inside the guest :). > Just following along on this thread, and wanted to add a few of my experiences with this scenario from the kernel target perspective.. 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. 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. 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) 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. --nab _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
