linux-kernel - Re: [PATCH] bsg : Add support for io vectors in bsg

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Date:	Wed, 09 Jan 2008 09:11:18 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	pw@....edu
Cc:	tomof@....org, deepakrc@...il.com, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org, fujita.tomonori@....ntt.co.jp
Subject: Re: [PATCH] bsg : Add support for io vectors in bsg

On Tue, 8 Jan 2008 17:09:18 -0500
Pete Wyckoff <pw@....edu> wrote:

> tomof@....org wrote on Sat, 05 Jan 2008 14:01 +0900:
> > From: Deepak Colluru <deepakrc@...il.com>
> > Subject: [PATCH] bsg : Add support for io vectors in bsg
> > Date: Fri, 4 Jan 2008 21:47:34 +0530 (IST)
> > 
> > > From: Deepak Colluru <deepakrc@...il.com>
> > > 
> > > Add support for io vectors in bsg.
> > > 
> > > Signed-off-by: Deepak Colluru <deepakrc@...il.com>
> > > ---
> > >   bsg.c |   52 +++++++++++++++++++++++++++++++++++++++++++++++++---
> > >   1 file changed, 49 insertions(+), 3 deletions(-)
> > 
> > Thanks, but I have to NACK this.
> > 
> > You can find the discussion about bsg io vector support and a similar
> > patch in linux-scsi archive. I have no plan to support it since it
> > needs the compat hack.
> 
> You may recall this is one of the patches I need to use bsg with OSD
> devices.  OSDs overload the SCSI buffer model to put mulitple fields
> in dataout and datain.  Some is user data, but some is more
> logically created by a library.  Memcpying in userspace to wedge all
> the segments into a single buffer is painful, and is required both
> on outgoing and incoming data buffers.
> 
> There are two approaches to add iovec to bsg.
> 
> 1.  Define a new sg_iovec_v4 that uses constant width types.  Both
>     32- and 64-bit userspace would hand arrays of this to the kernel.
> 
>     struct sg_v4_iovec {
> 	    __u64 iov_base;
> 	    __u32 iov_len;
> 	    __u32 __pad1;
>     };
> 
>     Old patch here:  http://article.gmane.org/gmane.linux.scsi/30461/

As I said before, I don't think that inventing a new "iovec" is a good
idea. sgv3 use the common "iovec". In addition, sg_io_v4 can be used
by other OSes like sg_io_v3.


> 2.  Do as Deepak has done, using the existing sg_iovec, but then
>     also work around the compat issue.  Old v3 sg_iovec is:
> 
>     typedef struct sg_iovec /* same structure as used by readv() Linux system */
>     {                       /* call. It defines one scatter-gather element. */
> 	void __user *iov_base;      /* Starting address  */
> 	size_t iov_len;             /* Length in bytes  */
>     } sg_iovec_t;
> 
>     Old patch here:  http://article.gmane.org/gmane.linux.scsi/30460/
> 
> I took another look at the compat approach, to see if it is feasible
> to keep the compat handling somewhere else, without the use of #ifdef
> CONFIG_COMPAT and size-comparison code inside bsg.c.  I don't see how.
> The use of iovec is within a write operation on a char device.  It's
> not amenable to a compat_sys_ or a .compat_ioctl approach.
> 
> I'm partial to #1 because the use of architecture-independent fields
> matches the rest of struct sg_io_v4.  But if you don't want to have
> another iovec type in the kernel, could we do #2 but just return
> -EINVAL if the need for compat is detected?  I.e. change
> dout_iovec_count to dout_iovec_length and do the math?

If you are ok with removing the write/read interface and just have
ioctl, we could can handle comapt stuff like others do. But I think
that you (OSD people) really want to keep the write/read
interface. Sorry, I think that there is no workaround to support iovec
in bsg.
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