linux-kernel - Re: [PATCH v10 2/5] powerpc/vdso: Prepare for switching VDSO to generic C implementation.

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Date:   Wed, 5 Aug 2020 18:51:44 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, nathanl@...ux.ibm.com,
        anton@...abs.org, linux-arch@...r.kernel.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, luto@...nel.org, tglx@...utronix.de,
        vincenzo.frascino@....com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v10 2/5] powerpc/vdso: Prepare for switching VDSO to
 generic C implementation.

Hi Again,

Le 05/08/2020 à 16:03, Segher Boessenkool a écrit :
> Hi!
> 
> On Wed, Aug 05, 2020 at 07:09:23AM +0000, Christophe Leroy wrote:
>> +/*
>> + * The macros sets two stack frames, one for the caller and one for the callee
>> + * because there are no requirement for the caller to set a stack frame when
>> + * calling VDSO so it may have omitted to set one, especially on PPC64
>> + */
> 
> If the caller follows the ABI, there always is a stack frame.  So what
> is going on?

Looks like it is not the case. See discussion at 
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/2a67c333893454868bbfda773ba4b01c20272a5d.1588079622.git.christophe.leroy@c-s.fr/

Seems like GCC uses the redzone and doesn't set a stack frame. I guess 
it doesn't know that the inline assembly contains a function call so it 
doesn't set the frame.


Christophe