• Michael Ellerman's avatar
    powerpc: Introduce infrastructure for feature sections with alternatives · fac23fe4
    Michael Ellerman authored
    The current feature section logic only supports nop'ing out code, this means
    if you want to choose at runtime between instruction sequences, one or both
    cases will have to execute the nop'ed out contents of the other section, eg:
    
    BEGIN_FTR_SECTION
    	or	1,1,1
    END_FTR_SECTION_IFSET(FOO)
    BEGIN_FTR_SECTION
    	or	2,2,2
    END_FTR_SECTION_IFCLR(FOO)
    
    and the resulting code will be either,
    
    	or	1,1,1
    	nop
    
    or,
    	nop
    	or	2,2,2
    
    For small code segments this is fine, but for larger code blocks and in
    performance criticial code segments, it would be nice to avoid the nops.
    This commit starts to implement logic to allow the following:
    
    BEGIN_FTR_SECTION
    	or	1,1,1
    FTR_SECTION_ELSE
    	or	2,2,2
    ALT_FTR_SECTION_END_IFSET(FOO)
    
    and the resulting code will be:
    
    	or	1,1,1
    or,
    	or	2,2,2
    
    We achieve this by extending the existing FTR macros. The current feature
    section semantic just becomes a special case, ie. if the else case is empty
    we nop out the default case.
    
    The key limitation is that the size of the else case must be less than or
    equal to the size of the default case. If the else case is smaller the
    remainder of the section is nop'ed.
    
    We let the linker put the else case code in with the rest of the text,
    so that relative branches from the else case are more likley to link,
    this has the disadvantage that we can't free the unused else cases.
    
    This commit introduces the required macro and linker script changes, but
    does not enable the patching of the alternative sections.
    
    We also need to update two hand-made section entries in reg.h and timex.h
    Signed-off-by: default avatarMichael Ellerman <michael@ellerman.id.au>
    Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
    fac23fe4
vmlinux.lds.S 4.45 KB