From: Alan Stern
Sent: 22 June 2021 14:29
...
Thought...
Is kmalloc(1, GFP_KERNEL) guaranteed to return a pointer into a cache line that will not be accessed by any other code? (This is slightly weaker than requiring a cache-line aligned pointer - but very similar.)
As I understand it, on architectures that do not have cache-coherent I/O, kmalloc is guaranteed to return a buffer that is cacheline-aligned and whose length is a multiple of the cacheline size.
Now, whether that buffer ends up being accessed by any other code depends on what your driver does with the pointer it gets from kmalloc. :-)
Thanks for the clarification.
Most of the small allocates in the usb stack are for transmits where it is only necessary to ensure a cache write-back.
I know there has been some confusion because one of the allocators can add a small header to every allocation. This can lead to unexpectedly inadequately aligned pointers. If it is updated when the preceding block is freed (as some user-space mallocs do) then it would need to be in a completely separate cache line.
David
- Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)