Re: [PATCH net-next 3/4] s390/diag: add diag26c support

Martin Schwidefsky
2017-06-21T15:30:44Z

Hi Dave,

On Mon, 19 Jun 2017 13:37:43 -0400 (EDT)
David Miller <davem@davemloft.net> wrote:

> From: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Date: Mon, 19 Jun 2017 17:34:25 +0200
>
> > We (as in the s390 guys) tend to add __packed to hardware and hypervisor
> > structures even if the attribute is not strictly necessary. Most of the
> > diagnose related structures look that way. Dunno if it is worth to change
> > them.
>
> It causes gcc to generate bad code on certain platforms (yes, probably not
> yours) and is in general something to avoid.
>
> Please do not use __packed unless absolutely necessary.
>
> > The diag26c struct needs to be aligned on a doubleword boundary, the
> > __aligned(8) is necessary.
>
> That's fine.
>
> > The __packed attribute is again superfluous but follows along the
> > lines of the other diag structures.
>
> Please remove it.

I looked at the various structures with the packed attribute in arch/s390
and found that we could remove a lot of them. As __packed also changes the
alignment of the structure removing the attribute from structures defined
in arch/s390/include/uapi/asm may cause trouble in user space. I stayed
away from the uapi headers.

For the rest of the code about 120 packed attributes could be removed, see:

https://git.kernel.org/pub/scm/linux/kernel/git/s390/linux.git/log/?h=packed-cleanup

bloat-o-meter found only a few functions where the removal of packed
makes a difference (gcc 7.1.0):

add/remove: 1/1 grow/shrink: 8/7 up/down: 240/-308 (-68)
function old new delta
pin_scb.isra - 200 +200
stp_sync_clock 982 1000 +18
stp_work_fn 416 420 +4
sca_ext_call_pending 290 294 +4
kvm_arch_vcpu_create 2008 2012 +4
get_vcpu_asce 1206 1210 +4
hw_perf_event_update 1738 1740 +2
clp_add_pci_device 1400 1402 +2
__clp_rescan 162 164 +2
stp_timing_state_show 138 134 -4
stp_timing_mode_show 134 130 -4
stp_ctn_type_show 134 130 -4
stp_time_offset_show 150 144 -6
ccw_device_accumulate_irb 1838 1832 -6
clp_misc_ioctl 1142 1102 -40
timing_alert_interrupt 158 114 -44
pin_scb 200 - -200
Total: Before=155168810, After=155168742, chg -0.00%

The code gets minimally better. I am not convinced yet that this is worth
the hassle. These structures are architecture specific and the s390 CPUs
are perfectly fine with unaligned accesses.

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.