linux-kernel - Re: [PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv

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Date:	Tue, 07 Dec 2010 11:22:55 +0200
From:	Avi Kivity <avi@...hat.com>
To:	David Sharp <dhsharp@...gle.com>
CC:	rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	mrubin@...gle.com, kvm-devel <kvm@...r.kernel.org>
Subject: Re: [PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv_hypercall

On 12/06/2010 10:38 PM, David Sharp wrote:
> On Sat, Dec 4, 2010 at 12:11 AM, Avi Kivity<avi@...hat.com>  wrote:
> >  On 12/04/2010 02:13 AM, David Sharp wrote:
> >>
> >>  Signed-off-by: David Sharp<dhsharp@...gle.com>
> >>  ---
> >>    arch/x86/kvm/trace.h |    8 ++++----
> >>    1 files changed, 4 insertions(+), 4 deletions(-)
> >>
> >>  diff --git a/arch/x86/kvm/trace.h b/arch/x86/kvm/trace.h
> >>  index a6544b8..ab41fb0 100644
> >>  --- a/arch/x86/kvm/trace.h
> >>  +++ b/arch/x86/kvm/trace.h
> >>  @@ -62,21 +62,21 @@ TRACE_EVENT(kvm_hv_hypercall,
> >>          TP_ARGS(code, fast, rep_cnt, rep_idx, ingpa, outgpa),
> >>
> >>          TP_STRUCT__entry(
> >>  -               __field(        __u16,          code            )
> >>  -               __field(        bool,           fast            )
> >>                  __field(        __u16,          rep_cnt         )
> >>                  __field(        __u16,          rep_idx         )
> >>                  __field(        __u64,          ingpa           )
> >>                  __field(        __u64,          outgpa          )
> >>  +               __field(        __u16,          code            )
> >>  +               __field(        bool,           fast            )
> >>          ),
> >>
> >
> >  Looks like a pessimisation.
> >
> >  Before: 24 bytes
> >  After: 32 bytes
> >
> >  (on a 64-bit machine, assuming no packing)
>
> This patch is predicated on packing the event structures. And since
> the ring buffer is 32-bit addressable, I don't attempt to improve
> alignment beyond 32-bit boundaries.

I don't understand this.  Can you elaborate?  What does "32-bit 
addressable" mean?  And "predicated on packing the event structures"?  
Is the structure __attribute__((packed)), or is it not?

-- 
error compiling committee.c: too many arguments to function

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