Hi Dave, On Mon, 19 Jun 2017 13:37:43 -0400 (EDT) David Miller <davem@xxxxxxxxxxxxx> wrote: > From: Martin Schwidefsky <schwidefsky@xxxxxxxxxx> > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
