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;
Subject: Re: [PATCH 1/3] lib/vdso: Delay mask application in do_hres()
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     linux-arch@vger.kernel.org,
        LAK <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-mips@vger.kernel.org,
        linux-kselftest@vger.kernel.org, catalin.marinas@arm.com,
        Will Deacon <will.deacon@arm.com>,
        Arnd Bergmann <arnd@arndb.de>, linux@armlinux.org.uk,
        Ralf Baechle <ralf@linux-mips.org>, paul.burton@mips.com,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        salyzyn@android.com, pcc@google.com, shuah@kernel.org,
        0x7f454c46@gmail.com, linux@rasmusvillemoes.dk,
        huw@codeweavers.com, sthotton@marvell.com, andre.przywara@arm.com,
        Andy Lutomirski <luto@kernel.org>
References: <20190624133607.GI29497@fuggles.cambridge.arm.com>
 <20190625161804.38713-1-vincenzo.frascino@arm.com>
 <alpine.DEB.2.21.1906251851350.32342@nanos.tec.linutronix.de>
 <alpine.DEB.2.21.1906260832470.32342@nanos.tec.linutronix.de>
From:   Vincenzo Frascino <vincenzo.frascino@arm.com>
Message-ID: <4ff43de4-fe51-eed4-a155-31a05edf2f11@arm.com>
Date:   Wed, 26 Jun 2019 10:25:20 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.7.0
MIME-Version: 1.0
In-Reply-To: <alpine.DEB.2.21.1906260832470.32342@nanos.tec.linutronix.de>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi Thomas,

On 26/06/2019 07:38, Thomas Gleixner wrote:
> On Tue, 25 Jun 2019, Thomas Gleixner wrote:
>> On Tue, 25 Jun 2019, Vincenzo Frascino wrote:
>>> do_hres() in the vDSO generic library masks the hw counter value
>>> immediately after reading it.
>>>
>>> Postpone the mask application after checking if the syscall fallback is
>>> enabled, in order to be able to detect a possible fallback for the
>>> architectures that have masks smaller than ULLONG_MAX.
>>
>> Right. This only worked on x86 because the mask is there ULLONG_MAX for all
>> VDSO capable clocksources, i.e. that ever worked just by chance.
> 
> But it's actually worse than that:
> 
>>> +		cycles &= vd->mask;
>>>  		if (cycles > last)
>>>  			ns += (cycles - last) * vd->mult;
>>>  		ns >>= vd->shift;
> 
> This is broken for any clocksource which can legitimately wrap around. The
> core timekeeping does the right thing:
> 
>      		 (cycles - last) & mask
> 
> That makes sure that a wraparound is correctly handled. With the above the
> wrap around would be ignored due to
> 
>      	    if (cycles > last)
> 

You are right. Thanks for spotting it.


...

-- 
Regards,
Vincenzo