linux-kernel - Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK

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Message-ID: <63a0f67d-fdb1-e2fc-5d4d-4f3cfbf86fd1@redhat.com>
Date:   Fri, 14 Sep 2018 15:06:46 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Matt Rickard <matt@...trans.com.au>,
        Stephen Boyd <sboyd@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        devel@...uxdriverproject.org,
        virtualization@...ts.linux-foundation.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI
 support

On 09/14/2018 03:05 PM, Thomas Gleixner wrote:
> On Fri, 14 Sep 2018, Florian Weimer wrote:
> 
>> On 09/14/2018 02:50 PM, Thomas Gleixner wrote:
>>> Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
>>> implementation, which extended the clockid switch case and added yet
>>> another slightly different copy of the same code.
>>>
>>> Especially the extended switch case is problematic as the compiler tends to
>>> generate a jump table which then requires to use retpolines.
>>
>> Does vDSO code really have to use retpolines?  It's in userspace, after all.
> 
> Unless you have IBRS/STIPB enabled, you need user space ratpoutine as well.

I don't think this is a consensus position, and it obviously depends on 
the (sub)architecture.

Thanks,
Florian