Re: [PATCH V2] libibverbs: Allow arbitrary int values for MTU

["Jeff Squyres (jsquyres)" <jsquyres-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>]

On Jul 17, 2013, at 12:06 AM, "Hefty, Sean" <sean.hefty-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> wrote:

> I don't remember.  Is it known how the mtu is communicated with the kernel?  

I hadn't looked at the kernel side yet; I was waiting for the userspace side to sort itself out first.

> Looking at kern-abi.h, the mtu fields are:
> 
> struct ibv_query_port_resp {
> 	__u8  max_mtu;
> 	__u8  active_mtu;
> 
> struct ibv_kern_qp_attr {
> 	__u32	path_mtu;
> 
> struct ibv_query_qp_resp {
> 	__u8  path_mtu;
> 
> struct ibv_modify_qp {
> 	__u8  path_mtu;
> 
> In most cases, we only have 8 bits available to/from the kernel.  (There are at least 16 bits of reserved space in these structures.)

Hmm.  16 bits is probably enough for the MTU values, but still, changing kern-abi.h will be problematic from an ABI perspective.  Do people care about the kernel ABI, or is that mainly a userspace issue?

-- 
Jeff Squyres
jsquyres-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org
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