Messages in this thread | | | Subject | Re: [PATCH net 1/4] virtchnl: Fix layout of RSS structures | From | "Samudrala, Sridhar" <> | Date | Sun, 28 Mar 2021 20:19:19 -0700 |
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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
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