Re: [Xen-devel] [PATCH 03/13] x86/paravirt: Convert native patch assembly code strings to macros

[Josh Poimboeuf <jpoimboe@xxxxxxxxxx>]

On Fri, Nov 17, 2017 at 08:10:13PM +0100, Juergen Gross wrote:
> On 17/11/17 19:07, Borislav Petkov wrote:
> > On Wed, Oct 04, 2017 at 10:58:24AM -0500, Josh Poimboeuf wrote:
> >> Convert the hard-coded native patch assembly code strings to macros to
> >> facilitate sharing common code between 32-bit and 64-bit.
> >>
> >> These macros will also be used by a future patch which requires the GCC
> >> extended asm syntax of two '%' characters instead of one when specifying
> >> a register name.
> >>
> >> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> >> ---
> >>  arch/x86/include/asm/special_insns.h | 24 ++++++++++++++++++++++++
> >>  arch/x86/kernel/paravirt_patch_32.c  | 21 +++++++++++----------
> >>  arch/x86/kernel/paravirt_patch_64.c  | 29 +++++++++++++++--------------
> >>  3 files changed, 50 insertions(+), 24 deletions(-)
> >>
> >> diff --git a/arch/x86/include/asm/special_insns.h 
> >> b/arch/x86/include/asm/special_insns.h
> >> index ac402c6fc24b..0549c5f2c1b3 100644
> >> --- a/arch/x86/include/asm/special_insns.h
> >> +++ b/arch/x86/include/asm/special_insns.h
> >> @@ -6,6 +6,30 @@
> >>  
> >>  #include <asm/nops.h>
> >>  
> >> +#ifdef CONFIG_X86_64
> >> +# define _REG_ARG1                        "%rdi"
> >> +# define NATIVE_IDENTITY_32               "mov %edi, %eax"
> > 
> > Yeah, that "identity" looks strange. How about NATIVE_NOOP and
> > NATIVE_NOOP_32 ?
> 
> Those are not NOPs. They return the identical value which was passed to
> them. So identity isn't a bad name after all.

Right, like the math identity function:

  https://en.wikipedia.org/wiki/Identity_function

> >> +# define NATIVE_USERGS_SYSRET64           "swapgs; sysretq"
> >> +#else
> >> +# define _REG_ARG1                        "%eax"
> >> +#endif
> >> +
> >> +#define _REG_RET                  "%" _ASM_AX
> >> +
> >> +#define NATIVE_ZERO                       "xor " _REG_ARG1 ", " _REG_ARG1
> > 
> > NATIVE_ZERO_OUT
> > 
> > I guess. NATIVE_ZERO reads like the native representation of 0 :-)
> 
> NATIVE_ZERO_ARG1?

On a slight tangent, does anybody know why it zeros the arg?

The only place it's used is here:

#if defined(CONFIG_PARAVIRT_SPINLOCKS)
DEF_NATIVE(pv_lock_ops, queued_spin_unlock,     NATIVE_QUEUED_SPIN_UNLOCK);
DEF_NATIVE(pv_lock_ops, vcpu_is_preempted,      NATIVE_ZERO);
#endif

Isn't that a bug?  Seems like it should _return_ zero.  Zeroing the arg
shouldn't have any effect.

If I'm right, we could call it NATIVE_FALSE.

-- 
Josh

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