linux-kernel - Re: [PATCH] x86/ioperm: Use atomic64_inc_return() in ksys

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Message-ID: <3f0da3c0-22e8-4a19-9537-ac9c92b03174@zytor.com>
Date: Sat, 26 Oct 2024 16:38:24 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH] x86/ioperm: Use atomic64_inc_return() in ksys_ioperm()

On 10/26/24 16:28, H. Peter Anvin wrote:
> 
> By any sane definition, the constraint should actually be an input 
> constraint on the frame pointer itself; something like:
> 
> #define ASM_CALL_CONSTRAINT "r" (__builtin_frame_address(0))
> 
> ... except that "r" really should be a %rbp constraint, but %rbp doesn't 
> seem to have a constraint letter. At least gcc 14.2 seems to do the 
> right thing anyway, though: __builtin_frame_address(0) seems to force a 
> frame pointer to have been created (even with -fomit-frame-pointer 
> specified, and in a leaf function), and the value is always passed in 
> %rbp (because why on Earth would it do it differently, when it is 
> sitting right there?)
> 
cl
This also matches the "tell the compiler [and programmer] what we 
actually mean" issue that you have mentioned in other contexts.

Anyway, here is a simple test case that can be used to verify that this 
construct does indeed work; at least with gcc 14.2.1 and clang 18.1.8 
(the ones I ran a very quick test on).

It's simple enough that it is pretty straightforward to mess around with 
various modifications. So far I haven't been able to trip up the 
compilers this way.

	-hpa

View attachment "fp.c" of type "text/x-csrc" (541 bytes)