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SubjectRe: [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@redhat.com> wrote:
> > On 12/04/2010 02:13 AM, David Sharp wrote:
> >>
> >> Signed-off-by: David Sharp<dhsharp@google.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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