Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 31 Dec 2019 20:25:00 -0800
From:   Paul Burton <paulburton@kernel.org>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Christian Brauner <christian.brauner@canonical.com>
Subject: Re: [PATCH] mips: vdso: conditionalize 32-bit time functions on
 COMPAT_32BIT_TIME
Message-ID: <20200101042500.zmhldrthr3l45trq@lantea.localdomain>
References: <CAHmME9o==nBONywVgSjsmjs2H_A8itgmwibqzPmvivcSocKWRQ@mail.gmail.com>
 <CAK8P3a11g-UXcYdudDtp0TWCQAfotpc-63BqYwn-a9LDxV-b+Q@mail.gmail.com>
 <CAHmME9pnBtjJ86gsWgK8DCYKo_HFpyViHoGpJPTrYBONT01YVA@mail.gmail.com>
 <CAK8P3a0NBuqDX63+920q7Q+yO2xCoSd0O7xUDJv6BBoco2kVOg@mail.gmail.com>
 <CAHmME9rnnoFwh=EHAgdQFM+c33D9mgCoVML_+d=Js=pXPnsxKQ@mail.gmail.com>
 <CAK8P3a1AYGbgf6mmL-863+PXPBQw3AAtp5wQPL5duLCONGhHuw@mail.gmail.com>
 <CAHmME9r2G4viiF4sYqpuYOnFCtjWpjugqenoEeUrsU6MGBeSuQ@mail.gmail.com>
 <CAHmME9rCCei_CL1evDYQ44Eu8erbjx_jDzbpKeXD4jTGBu8v8A@mail.gmail.com>
 <CAHmME9q0z4RBhy7AETo0sPt+SfV7=zFra0iyZ-Sm54c-WG6CVg@mail.gmail.com>
 <20200101041056.xionott76qbrffih@lantea.localdomain>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20200101041056.xionott76qbrffih@lantea.localdomain>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi Jason,

On Tue, Dec 31, 2019 at 08:10:56PM -0800, Paul Burton wrote:
> I'm still not quite sure *why* this happens; perhaps GCC just decides it
> doesn't need to save & restore $gp/$28 when it spots that it's being
> "used" for __current_thread_info (even though that's never actually
> referenced in the VDSO)?

Ah:

> After defining a global register variable, for the current compilation
> unit:
> 
> - If the register is a call-saved register, call ABI is affected: the
>   register will not be restored in function epilogue sequences after
>   the variable has been assigned. Therefore, functions cannot safely
>   return to callers that assume standard ABI.

https://gcc.gnu.org/onlinedocs/gcc/Global-Register-Variables.html

That makes sense then. What doesn't make sense is how this ever
worked. A question for another day...

Thanks,
    Paul