From: Segher Boessenkool > Sent: 26 January 2023 17:19 > > On Thu, Jan 26, 2023 at 12:09:53AM +1100, Michael Ellerman wrote: > > Andrew Donnellan <ajd@xxxxxxxxxxxxx> writes: > > > A number of structures and buffers passed to PKS hcalls have alignment > > > requirements, which could on occasion cause problems: > > > > > > - Authorisation structures must be 16-byte aligned and must not cross a > > > page boundary > > > > > > - Label structures must not cross page boundaries > > > > > > - Password output buffers must not cross page boundaries > > > > > > Round up the allocations of these structures/buffers to the next power of > > > 2 to make sure this happens. > > > > It's not the *next* power of 2, it's the *nearest* power of 2, including > > the initial value if it's already a power of 2. > > It's not the nearest either, the nearest power of two to 65 is 64. You > could say "but, round up" to which I would say "round?" :-P > > "Adjust the allocation size to be the smallest power of two greater than > or equal to the given size." > > "Pad to a power of two" in shorthand. "Padded to a power of two if > necessary" if you want to emphasise it can be a no-op. Changing the size to kzalloc() doesn't help. The alignment depends on the allocator and is only required to have a relatively small alignment (ARCH_MINALIGN?) regardless of the size. IIRC one of the allocators adds a small header to every item. It won't return 16 byte aligned items at all. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
