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

[Juergen Gross]

On 17/11/17 20:42, Josh Poimboeuf wrote:
> 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?

Why are _you_ asking? You've introduced it.

> 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.

Right. Before that patch it _did_ return zero instead of zeroing arg1.

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

I'd prefer NATIVE_ZERO, as it will be usable for non-boolean cases, too.


Juergen