[openib-general] [PATCH 4/7] IB/ipath - performance improvements via mmap of queues

[Ralph Campbell]

On Mon, 2006-08-14 at 12:37 -0700, Roland Dreier wrote:
>     ralphc> This doesn't break compatibility.  uverbs_cmd.c
>     ralphc> ib_uverbs_resize_cq() allocates a struct
>     ralphc> ib_uverbs_resize_cq_resp on the stack but only reads the
>     ralphc> first element in.  The structure change isn't really
>     ralphc> needed at all since the INIT_UDATA() macro gets the start
>     ralphc> of driver_data from the struct ib_uverbs_resize_cq.  The
>     ralphc> change to ib_uverbs_resize_cq_resp just matches the
>     ralphc> structure change used by libipathverbs to initialize
>     ralphc> ib_uverbs_resize_cq.response.
> 
> Am I missing something?  Think about the case of old libibverbs, new
> kernel.  libibverbs allocates a 4-byte response structure and passes
> the pointer to that to the kernel.  The kernel allocates an 8-byte
> response structure and copies it back to userspace.  And the 4 bytes
> after the userspace response structure get zeroed.
> 
>  - R.

No, I was missing something :-)
You are correct. The structure change was needed
to get the alignment correct for returning a u64
after the struct ib_uverbs_resize_cq_resp.
I can avoid the incompatibility a number of ways:
1) change ib_uverbs_resize_cq() to only copyout resp.cqe.
2) change ib_ipath to ALIGN the udata->outbuf address.
3) define two resp structures, check the version, copyout the
   right one.
Seems like #1 is the simplest to me.