Hi Samudrala,
On Fri, Mar 26, 2021 at 11:45 PM Samudrala, Sridhar
wrote:
> On 3/26/2021 1:06 AM, Geert Uytterhoeven wrote:
> > On Thu, Mar 25, 2021 at 11:29 PM Tony Nguyen
> > wrote:
> > From: Norbert Ciosek
> >
> > Remove padding from RSS structures. Previous layout
> > could lead to unwanted compiler optimizations
> > in loops when iterating over key and lut arrays.
> >
> > From an earlier private conversation with Mateusz, I understand the real
> > explanation is that key[] and lut[] must be at the end of the
> > structures, because they are used as flexible array members?
> >
> > Fixes: 65ece6de0114 ("virtchnl: Add missing explicit padding to structures")
> > Signed-off-by: Norbert Ciosek
> > Tested-by: Konrad Jankowski
> > Signed-off-by: Tony Nguyen
> >
> > --- a/include/linux/avf/virtchnl.h
> > +++ b/include/linux/avf/virtchnl.h
> > @@ -476,7 +476,6 @@ struct virtchnl_rss_key {
> > u16 vsi_id;
> > u16 key_len;
> > u8 key[1]; /* RSS hash key, packed bytes */
> > - u8 pad[1];
> > };
> >
> > VIRTCHNL_CHECK_STRUCT_LEN(6, virtchnl_rss_key);
> > @@ -485,7 +484,6 @@ struct virtchnl_rss_lut {
> > u16 vsi_id;
> > u16 lut_entries;
> > u8 lut[1];/* RSS lookup table */
> > - u8 pad[1];
> > };
> >
> > If you use a flexible array member, it should be declared without a size,
> > i.e.
> >
> > u8 key[];
> >
> > Everything else is (trying to) fool the compiler, and leading to undefined
> > behavior, and people (re)adding explicit padding.
>
> This header file is shared across other OSes that use C++ that doesn't support
> flexible arrays. So the structures in this file use an array of size 1 as a
> last
> element to enable variable sized arrays.
I don't think it is accepted practice to have non-Linux-isms in
include/*linux*/avf/virtchnl.h header files. Moreover, using a size
of 1 is counter-intuitive for people used to Linux kernel development,
and may lead to off-by-one errors in calculation of sizes.
If you insist on ignoring the above, this definitely deserves a
comment next to the member's declaration.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
