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Re: [PATCH v2.1 RFC 17/12] xen/trace: Drop cycles parameter

  • To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Wed, 22 Sep 2021 09:03:56 +0200
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  • Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Meng Xu <mengxu@xxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Wed, 22 Sep 2021 07:04:07 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 21.09.2021 22:08, Andrew Cooper wrote:
> Not a patch, but an RFC idea for one...
> It occurred to me that the cycles parameter from __trace_var() and
> friends is pointless, as the cycles bit is encoded in the top bit of the
> event field anyway, and passed in the adjacent parameter.
> Dropping the cycles parameter results in +85/-1029 (-944) net change.
> The common change in callers is e.g. from:
> lea    0x3c(%rsp),%rcx
> mov    $0x4,%edx
> mov    $0x1,%esi
> mov    $0x10f002,%edi
> mov    %ebp,0x3c(%rsp)
> callq  ffff82d04022ea20 <__trace_var>
> to this:
> lea    0x3c(%rsp),%rdx
> mov    $0x4,%esi
> mov    $0x8010f002,%edi
> mov    %ebp,0x3c(%rsp)
> callq  ffff82d04022ea20 <__trace_var>
> Just sprinkling "| TRC_HD_CYCLE_FLAG" over the place makes things a
> little bit unwieldy.
> Instead, I was thinking of implementing trace() (and a thin trace_time()
> wrapper) mirroring the "new API" in patch 14.  Half of the trace_var()
> users should be __trace_var() already because of living inside a
> tb_init_done conditional, and the rest are actually opencoded TRACE()
> taking no extra data.
> Thoughts?

Sounds quite reasonable to me.




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