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

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Date:	Mon, 6 Dec 2010 12:38:26 -0800
From:	David Sharp <dhsharp@...gle.com>
To:	Avi Kivity <avi@...hat.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 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 have a truly marvellous patch that fixes the bug which this
> signature is too narrow to contain.
>
>