• Jeremy Fitzhardinge's avatar
    x86/paravirt: add register-saving thunks to reduce caller register pressure · ecb93d1c
    Jeremy Fitzhardinge authored
    Impact: Optimization
    
    One of the problems with inserting a pile of C calls where previously
    there were none is that the register pressure is greatly increased.
    The C calling convention says that the caller must expect a certain
    set of registers may be trashed by the callee, and that the callee can
    use those registers without restriction.  This includes the function
    argument registers, and several others.
    
    This patch seeks to alleviate this pressure by introducing wrapper
    thunks that will do the register saving/restoring, so that the
    callsite doesn't need to worry about it, but the callee function can
    be conventional compiler-generated code.  In many cases (particularly
    performance-sensitive cases) the callee will be in assembler anyway,
    and need not use the compiler's calling convention.
    
    Standard calling convention is:
    	 arguments	    return	scratch
    x86-32	 eax edx ecx	    eax		?
    x86-64	 rdi rsi rdx rcx    rax		r8 r9 r10 r11
    
    The thunk preserves all argument and scratch registers.  The return
    register is not preserved, and is available as a scratch register for
    unwrapped callee code (and of course the return value).
    
    Wrapped function pointers are themselves wrapped in a struct
    paravirt_callee_save structure, in order to get some warning from the
    compiler when functions with mismatched calling conventions are used.
    
    The most common paravirt ops, both statically and dynamically, are
    interrupt enable/disable/save/restore, so handle them first.  This is
    particularly easy since their calls are handled specially anyway.
    
    XXX Deal with VMI.  What's their calling convention?
    Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
    ecb93d1c
paravirt.c 13.2 KB