Re: [PATCH v9 04/10] x86: refcount: prevent gcc distortions

Nadav Amit
2018-10-04T20:06:02Z

at 12:33 PM, H. Peter Anvin <hpa@zytor.com> wrote:

> On 10/04/18 02:16, Ingo Molnar wrote:
>> * hpa@zytor.com <hpa@zytor.com> wrote:
>>
>>> Ingo: I wasn't talking necessarily about the specifics of each bit, but rather the general
>>> concept about being able to use macros in inlines...
>>
>> Ok, agreed about that part - and some of the patches did improve readability.
>>
>> Also, the 275 lines macros.s is a lot nicer than the 4,200 lines macros.S.
>>
>> Also, I'm not against using workarounds when the benefits are larger than the costs, but I am
>> against *hiding* the fact that these are workarounds and that for some of them there are costs.
>
> Agreed, of course.
>
>>> I can send you something I have been working on in the background, but have been holding off
>>> on because of this, in the morning my time.
>>
>> BTW., I have applied most of the series to tip:x86/kbuild already, and will push them out later
>> today after some testing. I didn't apply the final 3 patches as they have dependencies, but
>> applied the basics and fixed up the changelogs.
>>
>> So you can rely on this.
>
> Wonderful.
>
> Here is the horrible code I mentioned yesterday. This is about
> implementing the immediate-patching framework that Linus and others have
> discussed (it helps both performance and kernel hardening):
>
> Warning: this stuff can cause serious damage to your eyes, and this is a
> just a small chunk of the whole mess; and relying on gas macros, as
> brain damaged as they are, really is much, much cleaner than not:
>
> https://na01.safelinks.protection.outlook.com/?url=http:%2F%2Fwww.zytor.com%2F~hpa%2Ffoo.S&amp;data=02%7C01%7Cnamit%40vmware.com%7C326f1a3beb4649df319508d62a3042fa%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C1%7C0%7C636742784111671122&amp;sdata=anYIOXzlSTXPQKttTBHjSQgapBmaO9gfibBF34ZlHeQ%3D&amp;reserved=0

Funny. Immediate-patching is what I was playing with when I encountered the
gcc issue. Performance got worse instead of improving (or at least staying
the same), because inlining got crazy.

Anyhow, wait for my soon-to-be-sent RFC in which I define a macro called
“call” (to reduce the retpoline overhead) before you talk about damage to
the eyes.