Re: [Qemu-devel] [Qemu-trivial] [PATCH] srp: Don't use QEMU_PACKED for single elements of a structured type

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Aug 15, 2012 at 05:59:26PM +0200, Stefan Weil wrote:
> Am 15.08.2012 16:16, schrieb Stefan Hajnoczi:
> >On Fri, Aug 10, 2012 at 10:03:27PM +0200, Stefan Weil wrote:
> >>QEMU_PACKED results in a MinGW compiler warning when it is
> >>used for single structure elements:
> >>
> >>warning: 'gcc_struct' attribute ignored
> >>
> >>Using QEMU_PACKED for the whole structure avoids the compiler warning
> >>without changing the memory layout.
> >Quick link for other reviewers:
> >http://gcc.gnu.org/onlinedocs/gcc-4.7.1/gcc/Type-Attributes.html#Type-Attributes
> >
> >>Signed-off-by: Stefan Weil <sw@weilnetz.de>
> >>---
> >>  hw/srp.h |    8 ++++----
> >>  1 files changed, 4 insertions(+), 4 deletions(-)
> >>
> >>diff --git a/hw/srp.h b/hw/srp.h
> >>index 3009bd5..5e0cad5 100644
> >>--- a/hw/srp.h
> >>+++ b/hw/srp.h
> >>@@ -177,13 +177,13 @@ struct srp_tsk_mgmt {
> >>      uint8_t    reserved1[6];
> >>      uint64_t   tag;
> >>      uint8_t    reserved2[4];
> >>-    uint64_t   lun QEMU_PACKED;
> >>+    uint64_t   lun;
> >>      uint8_t    reserved3[2];
> >>      uint8_t    tsk_mgmt_func;
> >>      uint8_t    reserved4;
> >>      uint64_t   task_tag;
> >>      uint8_t    reserved5[8];
> >>-};
> >>+} QEMU_PACKED;
> >Here I actually see a difference for the uint64_t task_tag field.
> >Previously it was not packed, now it is packed and because it has 4 *
> >uint8_t before it there will be a difference in layout.
> >
> >Looking at how QEMU accesses srp_tsk_mgmt, I think we're safe because we
> >never actually access task_tag?
> >
> >Ben: Any thoughts on this patch?
> >
> >Stefan
> 
> 4 * uint8_t + 4 bytes from the packed lun, so there is no change
> for task_tag, it's always on a 8 byte boundary!

Ah, yes, I see it now!  Glad we're switching to struct-level packing :).

Stefan