• Michael Ellerman's avatar
    powerpc: Add ppc_inst_next() · c5ff46d6
    Michael Ellerman authored
    In a few places we want to calculate the address of the next
    instruction. Previously that was simple, we just added 4 bytes, or if
    using a u32 * we incremented that pointer by 1.
    
    But prefixed instructions make it more complicated, we need to advance
    by either 4 or 8 bytes depending on the actual instruction. We also
    can't do pointer arithmetic using struct ppc_inst, because it is
    always 8 bytes in size on 64-bit, even though we might only need to
    advance by 4 bytes.
    
    So add a ppc_inst_next() helper which calculates the location of the
    next instruction, if the given instruction was located at the given
    address. Note the instruction doesn't need to actually be at the
    address in memory.
    
    Although it would seem natural for the value to be passed by value,
    that makes it too easy to write a loop that will read off the end of a
    page, eg:
    
    	for (; src < end; src = ppc_inst_next(src, *src),
    			  dest = ppc_inst_next(dest, *dest))
    
    As noticed by Christophe and Jordan, if end is the exact end of a
    page, and the next page is not mapped, this will fault, because *dest
    will read 8 bytes, 4 bytes into the next page.
    
    So value is passed by reference, so the helper can be careful to use
    ppc_inst_read() on it.
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Reviewed-by: default avatarJordan Niethe <jniethe5@gmail.com>
    Link: https://lore.kernel.org/r/20200522133318.1681406-1-mpe@ellerman.id.au
    c5ff46d6
feature-fixups.c 23.9 KB