linux-kernel - Re: [RFC PATCH v1 1/2] selftests/x86: sysret

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Date:   Wed, 15 Feb 2023 10:29:37 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Brian Gerst <brgerst@...il.com>,
        Borislav Petkov <bp@...en8.de>, Shuah Khan <shuah@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        x86 Mailing List <x86@...nel.org>,
        Linux Kselftest Mailing List 
        <linux-kselftest@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v1 1/2] selftests/x86: sysret_rip: Handle syscall in a
 FRED system

On 15/02/2023 9:17 am, Andrew Cooper wrote:
> On 26/01/2023 8:08 pm, Ammar Faizi wrote:
>> On Mon, Jan 23, 2023 at 05:40:23PM -0800, H. Peter Anvin wrote:
>>> So as per Andrew's comment, add:
>>>
>>> register void * rsp asm("%rsp");
>>>
>>> ...
>>>
>>> "+r" (rsp)	/* clobber the redzone */
>>>
>>> ... as the right way to avoid redzone problems.
>> I played with this more. I found something wrong with this. This doesn't
>> work for me. The compiler still uses red zone despite I use "+r" (rsp).
>>
>> What did I do wrong?
> Well this is a fine mess...
>
> https://godbolt.org/z/MaPM7s8qr does the right thing, but is now
> contrary to the prior discussion regarding calls in asm, which concluded
> that the "+r"(rsp) was the way to go.
>
> Furthermore GCC regressed in 9.0 and emits:
>
>   warning: listing the stack pointer register 'rsp' in a clobber list is
> deprecated [-Wdeprecated]
>
> which might be the intention of the developers, but is wrong seeing as
> this is the only way to say "I modify the redzone" to the compiler...

I've opened https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108799

~Andrew