On Sat, Nov 10, 2018 at 2:36 AM Arnd Bergmann <arnd@xxxxxxxx> wrote: > > On Fri, Nov 9, 2018 at 6:13 PM Sunil Kovvuri <sunil.kovvuri@xxxxxxxxx> wrote: > > On Fri, Nov 9, 2018 at 4:32 PM Arnd Bergmann <arnd@xxxxxxxx> wrote: > > > On Fri, Nov 9, 2018 at 5:21 AM Sunil Kovvuri <sunil.kovvuri@xxxxxxxxx> wrote: > > > > > > > Since b is aligned to four bytes, you get padding between a and b. > > > On top of that, you also get padding after c to make the size of > > > structure itself be a multiple of its alignment. For interfaces, we > > > should avoid both kinds of padding. This can be done by marking > > > members as __packed (usually I don't recommend that), by > > > changing the size of members, or by adding explicit 'reserved' > > > fields in place of the padding. > > > > > > > > I also noticed a similar problem in struct mbox_msghdr. Maybe > > > > > use the 'pahole' tool to check for this kind of padding in the > > > > > API structures. > > > > Got your point now and agree that padding has to be avoided. > > But this is a big change and above pointed structure is not > > the only one as this applies to all structures in the file. > > > > Would it be okay if I submit a separate patch after this series > > addressing all structures ? > > It depends on how you want to address it. If you want to > change the structure layout, then I think it would be better > integrated into the series as that is an incompatible interface > change. If you just want to add reserved members to make > the padding explicit, that could be a follow-up. > > Arnd Upon further thought i think padding is needed here but adding reserved fields is also not enough. Probably i will have to align all struct elements to 64bit with __attribute__ ((aligned(8))) to avoid unaligned access faults when device is attached to guest. VFIO maps BARs as device memory, so irrespective of whatever mapping i do in driver, reads/writes to the mailbox region might result in unaligned access fault. Checking if there are other ways of solving it. Thanks, Sunil.
