Re: [PATCH v3 15/18] bug/x86/arm: Align bug_frames sections.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Sep 13, 2016 at 03:21:04AM -0600, Jan Beulich wrote:
> >>> On 11.09.16 at 22:35, <konrad.wilk@oracle.com> wrote:
> > Furthermore on x86 the bloat-o-meter detects that with this
> > change:
> > 
> > [konrad@char xen]$ ~/linux/scripts/bloat-o-meter xen-syms xen-syms.align4
> > add/remove: 0/0 grow/shrink: 3/14 up/down: 115/-1497 (-1382)
> > function                                     old     new   delta
> > mod_l1_entry                                1490    1587     +97
> > p2m_switch_domain_altp2m_by_id               520     533     +13
> > p2m_switch_vcpu_altp2m_by_id                 453     458      +5
> > machine_kexec                                263     261      -2
> > hvm_do_IRQ_dpci                              244     242      -2
> > sh_page_fault__guest_4                      8252    8192     -60
> > sh_audit_gw                                 1529    1462     -67
> > validate_gl3e                                337     264     -73
> > validate_gl4e                                449     375     -74
> > p2m_altp2m_lazy_copy                         730     652     -78
> > set_typed_p2m_entry                         1346    1259     -87
> > virt_to_xen_l2e                              491     365    -126
> > sh_x86_emulate_write__guest_4                443     288    -155
> > p2m_mem_access_check                        1733    1576    -157
> > sh_x86_emulate_cmpxchg__guest_4              512     349    -163
> > __get_gfn_type_access                        709     542    -167
> > map_pages_to_xen                            4430    4144    -286
> > Total: Before=1974033, After=1972651, chg -0.07%
> > 
> > We end up making the binary file a bit smaller.
> 
> I'm fine with the change, but I'm having a very hard time buying the
> above: The change you do is to code used only in assembly files _and_

That is indeed fishy. I re-ran the test and got:

 ~/linux/scripts/bloat-o-meter /tmp/xen-syms.orig /tmp/xen-syms.p2malign 
add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0)
function                                     old     new   delta

which is more inline with what we expect.

> is not affecting .text (and I assume the sizes above are .text ones),
> yet all the functions listed above live in C ones. I can't help thinking
> that there must be something else going on, or that we had an actual
> problem before this.

I am not sure what I managed to screw up.
> 
> > --- a/xen/include/asm-x86/bug.h
> > +++ b/xen/include/asm-x86/bug.h
> > @@ -98,6 +98,7 @@ extern const struct bug_frame __start_bug_frames[],
> >      .popsection
> >  
> >      .pushsection .bug_frames.\type, "a", @progbits
> > +        .p2align 2
> >          .L\@bf:
> >          .long (.L\@ud - .L\@bf) + \
> >                 ((\line >> BUG_LINE_LO_WIDTH) << BUG_DISP_WIDTH)
> 
> This ought to be accompanied by removing the enforcing of alignment
> in xen.lds.S.

/me nods. Done!
> 
> Jan
> 

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