Re: RFC [patch] README.passthrough v1 (2nd send)

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On Thu, 2010-08-19 at 16:23 +1000, Mark Harvey wrote:
> On Thu, Aug 19, 2010 at 11:01 AM, Nicholas A. Bellinger
> > So the main difference from the perspective of a userspace space app is
> > going from using include/scsi/sg.h:struct sg_io_hdr to using
> > include/linux/bsg.h:struct sg_io_v4.  On modern kernel code, under the
> > hood in the kernel both drivers/scsi/sg.c and block/bsg.c use struct
> > request based I/O via blk_get_request() to obtain a struct request, and
> > blk_execute_rq_nowait() to asychronously queue the I/O to the underlying
> > backstore.
> >
> > BSG exposes more raw functionality (but certaily not all) of the modern
> > Linux Block layer code.  This includes the ability to do BIDI commands
> > and support for legacy SG_IO ioctls in block/bsg.c:bsg_ioctl().
> >
> > Note that not all of the the structure members in struct sg_io_v4 can
> > safely be used with current mainline code.  For example, having a 32-bit
> > userspace passing up 32-bit pointers currently need a patch to require
> > proper conversion to 64-bit struct scatterlist memory.  The first half
> > of this patch is here:
> >
> > http://groups.google.com/group/linux-iscsi-target-dev/browse_thread/thread/6b9f8a08f78067dc/6330a5897203854a?lnk=gst&q=BSG#6330a5897203854a
> >
> > This issue came up again between Tomo-san and myself last week, and we
> > agreed that the patch to make 32-bit user + 64-bit kernel work with BSG
> > scatterlists would not be pretty, but that it does need to be fixed in
> > order to properly support the QEMU-KVM SCSI-BSG MegaSAS SGL passthrough.
> > This is the code that has been being going in parallel with the recent
> > multi-fabric TCM_Loop Virtual SCSI LLD develpments for STGT, which can
> > be found here to give more of an idea how BSG functions.
> >
> > http://git.kernel.org/?p=virt/kvm/nab/qemu-kvm.git;a=blob;f=hw/scsi-bsg.c;hb=refs/heads/scsi-bsg
> >
> > Best,
> >
> > --nab
> 
> OK, I'm having some problems with 1.0.7 build attempting to configure
> a bsg device.
> OS is SLES 11 x86_64
> Linux mhvtl5 2.6.27.19-5-default #1 SMP 2009-02-28 04:40:21 +0100
> x86_64 x86_64 x86_64 GNU/Linux
> 
> # lsscsi -g
> [3:0:1:0]    tape    IBM      ULT3580-TD5      550V  /dev/st0  /dev/sg2
> 
> # ls -l /dev/bsg/
> total 0
> crw-rw---- 1 root root 253,  0 2010-08-15 13:15 0:0:0:0
> crw-rw---- 1 root root 253,  1 2010-08-15 13:15 2:0:0:0
> crw-rw---- 1 root root 253, 10 2010-08-17 12:02 3:0:0:0
> crw-rw---- 1 root root 253,  2 2010-08-17 12:02 3:0:1:0
> crw-rw---- 1 root root 253,  7 2010-08-17 12:02 3:0:10:0
> crw-rw---- 1 root root 253,  8 2010-08-17 12:02 3:0:11:0
> crw-rw---- 1 root root 253,  9 2010-08-17 12:02 3:0:12:0
> crw-rw---- 1 root root 253,  3 2010-08-17 12:02 3:0:2:0
> crw-rw---- 1 root root 253,  4 2010-08-17 12:02 3:0:3:0
> crw-rw---- 1 root root 253,  5 2010-08-17 12:02 3:0:4:0
> crw-rw---- 1 root root 253, 11 2010-08-17 12:02 3:0:8:0
> crw-rw---- 1 root root 253,  6 2010-08-17 12:02 3:0:9:0
> 
> # tgtadm --op new --mode target --tid 1 -T iqn.2010-08.com.nbusyd:mh
> # tgtadm --op new --mode logicalunit --tid 1 --lun 1 --bstype=bsg
> --device-type=pt -b /dev/bsg/3:0:1:0
> tgtadm: invalid request
> 
> All the syslog shows is:
> tgtd: bs_sg_open(361) Not recognized /dev/bsg/3:0:1:0 as an SG device
> 

Hmmmmm, so it appears that usr/bs_sg.c:chk_sg_device() is failing to
pick up the BSG device nodes here:

        /* Check for SG_IO major first.. */
        if (major(st.st_rdev) == SCSI_GENERIC_MAJOR)
                return 0;

        /* This is not yet defined in include/linux/major.h.. */
        if (major(st.st_rdev) == 254)
                return 1;

        return -1;
}

So it looks like the failure is attributed to my hardcoding of major 254
in the original STGT/BSG patch (note the comment above the conditional
check).  Here is a quick fix:

diff --git a/usr/bs_sg.c b/usr/bs_sg.c
index cda5cd2..a369570 100644
--- a/usr/bs_sg.c
+++ b/usr/bs_sg.c
@@ -289,7 +289,7 @@ static int chk_sg_device(char *path)
                return 0;
 
        /* This is not yet defined in include/linux/major.h.. */
-       if (major(st.st_rdev) == 254)
+       if ((major(st.st_rdev) == 254) || (major(st.st_rdev) == 253))
                return 1;
 
        return -1;

Best,

--nab

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