Re: [PATCH] bsg: iovec support

[Douglas Gilbert <dougg@torque.net>]

FUJITA Tomonori wrote:
> From: Douglas Gilbert <dougg@torque.net>
> Subject: Re: [PATCH] bsg: iovec support
> Date: Mon, 19 Mar 2007 08:56:39 -0400
> 
>> FUJITA Tomonori wrote:
>>> From: Pete Wyckoff <pw@osc.edu>
>>> Subject: [PATCH] bsg: iovec support
>>> Date: Thu, 1 Mar 2007 17:29:08 -0500
>>>
>>>> Support vectored IO as in SGv3.  The iovec structure uses explicit
>>>> sizes to avoid the need for compat conversion.
>>>>
>>>> Signed-off-by: Pete Wyckoff <pw@osc.edu>
>>>> ---
>>>>
>>>> My application definitely can take advantage of scatter/gather IO,
>>>> which is supported in sgv3 but not in the bsg implementation of sgv4.
>>>> I understand Tomo's concerns about code bloat and the need for
>>>> 32/64 compat translations, but this will make things much easier on
>>>> users of bsg who read or write out of multiple buffers in a single
>>>> SCSI operation.
>>> (snip)
>>>
>>>> + * Vector of address/length pairs, used when dout_iovec_count (or din_)
>>>> + * is non-zero.  In that case, dout_xferp is a list of struct sg_io_v4_vec
>>>> + * and dout_iovec_count is the number of entries in that list.  dout_xfer_len
>>>> + * is the total length of the list.  Note the use of u64 instead of a
>>>> + * native pointer to avoid compat issues, and padding to avoid structure
>>>> + * alignment problems.
>>>> + */
>>>> +struct sg_io_v4_vec {
>>>> +	__u64 iov_base;
>>>> +	__u32 iov_len;
>>>> +	__u32 __pad1;
>>>> +};
>>> I don't think that it's a good idea to add a new scatter/gather
>>> structure and export it to user space.
>> User space scatter gather is not a new feature.
>> It is defined and works in sg v3.
>>
>> It was also partially defined in sg v4 and dropped
>> out in the bsg implementation. I agree with Pete that
>> it should be put back.
> 
> I'm fine with supporting iovec (though I don't like it).

Tomo,
You don't need to support it if you don't want to.
So if din_iovec_count or dout_iovec_count are other
than zero, bsg can return an error.

By dropping those fields, other implementations are
precluded from supporting that feature.

>> Pete is also suggesting (shown above) a revised sg_io_vec
>> structure that uses a uint64_t for the pointer to simplify
>> 32, 64 bit thunking.
> 
> All I said is that it would be better to use the existing compat
> infrastructure (sg_build_iovec, sg_ioctl_trans, etc in
> fs/compat_ioctl.c) instead of adding another compat code.

Won't sg v4 make this even a bigger mess, at least
initially anyway?

Doug Gilbert