Re: [Qemu-devel] [PATCH 2/2] slirp: Don't mark struct ipq or struct ipasfrag as packed

[Daniel P . Berrangé]

On Tue, Jan 22, 2019 at 06:18:22PM +0000, Peter Maydell wrote:
> There is no reason to mark the struct ipq and struct ipasfrag as
> packed: they are naturally aligned anyway, and are not representing
> any on-the-wire packet format.  Indeed they vary in size depending on
> the size of pointers on the host system, because the 'struct qlink'
> members include 'void *' fields.
> 
> Dropping the 'packed' annotation fixes clang -Waddress-of-packed-member
> warnings and probably lets the compiler generate better code too.
> 
> The only thing we do care about in the layout of the struct is
> that the frag_link matches up with the ipf_link of the struct
> ipasfrag, as documented in the comment on that struct; assert
> at build time that this is the case.
> 
> Signed-off-by: Peter Maydell <address@hidden>
> ---
>  slirp/ip.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/slirp/ip.h b/slirp/ip.h
> index 243b6c8b249..20614f3b53e 100644
> --- a/slirp/ip.h
> +++ b/slirp/ip.h

Just here there's a misleading comment

  * size 28 bytes

that should be purged

> @@ -217,7 +217,7 @@ struct ipq {
>       uint8_t ipq_p;                  /* protocol of this fragment */
>       uint16_t        ipq_id;                 /* sequence id for reassembly */
>       struct  in_addr ipq_src,ipq_dst;
> -} QEMU_PACKED;
> +};
>  
>  /*
>   * Ip header, when holding a fragment.
> @@ -227,7 +227,10 @@ struct ipq {
>  struct       ipasfrag {
>       struct qlink ipf_link;
>       struct ip ipf_ip;
> -} QEMU_PACKED;
> +};
> +
> +QEMU_BUILD_BUG_ON(offsetof(struct ipq, frag_link) !=
> +                  offsetof(struct ipasfrag, ipf_link));
>  
>  #define ipf_off      ipf_ip.ip_off
>  #define ipf_tos      ipf_ip.ip_tos


Regards,
Daniel
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