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Message-ID: <0b57d2ab-14e0-14a0-adf3-3186fd2f5282@citrix.com>
Date:   Wed, 15 Feb 2023 09:17:23 +0000
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.7.1
From:   Andrew Cooper <Andrew.Cooper3@citrix.com>
Subject: Re: [RFC PATCH v1 1/2] selftests/x86: sysret_rip: Handle syscall in a
 FRED system
Content-Language: en-GB
To:     Ammar Faizi <ammarfaizi2@gnuweeb.org>,
        "H. Peter Anvin" <hpa@zytor.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Brian Gerst <brgerst@gmail.com>,
        Borislav Petkov <bp@alien8.de>, Shuah Khan <shuah@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        x86 Mailing List <x86@kernel.org>,
        Linux Kselftest Mailing List 
        <linux-kselftest@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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 <F554C5FE-5074-410A-B0B5-EFE983D57946@zytor.com>
 <Y88bhrDoPw5tOyKu@biznet-home.integral.gnuweeb.org>
 <509443c8-e0fd-935f-63d8-7264f5dd3c05@zytor.com>
 <20230124002625.581323-1-ammarfaizi2@gnuweeb.org>
 <20230124002625.581323-2-ammarfaizi2@gnuweeb.org>
 <8f5c24df-514d-5d89-f58f-ec8c3eb1e049@zytor.com>
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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...

~Andrew