Re: [PATCH net 1/4] virtchnl: Fix layout of RSS structures

[Samudrala, Sridhar]

On 3/27/2021 2:53 AM, Geert Uytterhoeven wrote:
> Hi Samudrala,
>
> On Fri, Mar 26, 2021 at 11:45 PM Samudrala, Sridhar
> <sridhar.samudrala@xxxxxxxxx> wrote:
> > On 3/26/2021 1:06 AM, Geert Uytterhoeven wrote:
> > > On Thu, Mar 25, 2021 at 11:29 PM Tony Nguyen <anthony.l.nguyen@xxxxxxxxx> wrote:
> > > From: Norbert Ciosek <norbertx.ciosek@xxxxxxxxx>
> > >
> > > 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 <norbertx.ciosek@xxxxxxxxx>
> > > Tested-by: Konrad Jankowski <konrad0.jankowski@xxxxxxxxx>
> > > Signed-off-by: Tony Nguyen <anthony.l.nguyen@xxxxxxxxx>
> > >
> > > --- 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.
Sure. We can add a comment indicating that these fields are used 
variable sized arrays.

Thanks
Sridhar