• David S. Miller's avatar
    sparc: Improve VDSO instruction patching. · 2f6c9bf3
    David S. Miller authored
    The current VDSO patch mechanism has several problems:
    
    1) It assumes how gcc will emit a function, with a register
       window, an initial save instruction and then immediately
       the %tick read when compiling vread_tick().
    
       There is no such guarantees, code generation could change
       at any time, gcc could put a nop between the save and
       the %tick read, etc.
    
       So this is extremely fragile and would fail some day.
    
    2) It disallows us to properly inline vread_tick() into the callers
       and thus get the best possible code sequences.
    
    So fix this to patch properly, with location based annotations.
    
    We have to be careful because we cannot do it the way we do
    patches elsewhere in the kernel.  Those use a sequence like:
    
    	1:
    	insn
    	.section	.whatever_patch, "ax"
    	.word		1b
    	replacement_insn
    	.previous
    
    This is a dynamic shared object, so that .word cannot be resolved at
    build time, and thus cannot be used to execute the patches when the
    kernel initializes the images.
    
    Even trying to use label difference equations doesn't work in the
    above kind of scheme:
    
    	1:
    	insn
    	.section	.whatever_patch, "ax"
    	.word		. - 1b
    	replacement_insn
    	.previous
    
    The assembler complains that it cannot resolve that computation.
    The issue is that this is contained in an executable section.
    
    Borrow the sequence used by x86 alternatives, which is:
    
    	1:
    	insn
    	.pushsection	.whatever_patch, "a"
    	.word		. - 1b, . - 1f
    	.popsection
    	.pushsection	.whatever_patch_replacements, "ax"
    	1:
    	replacement_insn
    	.previous
    
    This works, allows us to inline vread_tick() as much as we like, and
    can be used for arbitrary kinds of VDSO patching in the future.
    
    Also, reverse the condition for patching.  Most systems are %stick
    based, so if we only patch on %tick systems the patching code will
    get little or no testing.
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    2f6c9bf3
vdso2c.c 5.8 KB