| Message ID | 20210325223119.3991796-2-anthony.l.nguyen@intel.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 22f8b5df881e9f1302514bbbbbb8649c2051de55 |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | Intel Wired LAN Driver Updates 2021-03-25 | expand |
| Context | Check | Description |
|---|---|---|
| netdev/apply | success | Patch already applied to net |
| netdev/tree_selection | success | Clearly marked for net |
Hi Anthony, Norbert, Thanks for your patch! On Thu, Mar 25, 2021 at 11:29 PM Tony Nguyen <anthony.l.nguyen@intel.com> wrote: > From: Norbert Ciosek <norbertx.ciosek@intel.com> > > 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@intel.com> > Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com> > Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com> > --- 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. -- Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@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
Hi Samudrala, On Fri, Mar 26, 2021 at 11:45 PM Samudrala, Sridhar <sridhar.samudrala@intel.com> 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@intel.com> wrote: > > From: Norbert Ciosek <norbertx.ciosek@intel.com> > > > > 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@intel.com> > > Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com> > > Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com> > > > > --- 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
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@intel.com> 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@intel.com> wrote: >>> From: Norbert Ciosek <norbertx.ciosek@intel.com> >>> >>> 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@intel.com> >>> Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com> >>> Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com> >>> >>> --- 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
diff --git a/include/linux/avf/virtchnl.h b/include/linux/avf/virtchnl.h index 40bad71865ea..532bcbfc4716 100644 --- 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]; }; VIRTCHNL_CHECK_STRUCT_LEN(6, virtchnl_rss_lut);