linux-kernel - Re: [PATCH 00/16] arm/arm64: Workaround misprogrammed CNTFRQ

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Date:   Mon, 24 Jul 2017 13:19:59 +0100
From:   Will Deacon <will.deacon@....com>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 00/16] arm/arm64: Workaround misprogrammed CNTFRQ

On Fri, Jul 21, 2017 at 06:15:26PM +0100, Marc Zyngier wrote:
> It is an unfortunate situation that CNTFRQ{,_EL0} is often
> misprogrammed from the firmware side, leaving it up to the kernel to
> work around it. This is usually done by providing an alternative
> frequency in the Device Tree.
> 
> Unfortunately, CNTFRQ is accessible from EL0, giving userspace the
> wrong frequency, and potentially a different frequency per CPU, which
> is definitely not what you want. A possible workaround is to trap this
> into the kernel and to emulate it (together with the VDSO being
> disabled), and this is what this series is achieving.

Which userspace is actually affected by a broken CNTFRQ register? I suspect
most users will be more upset at losing their (perfectly functional) vDSO
acceleration than they are about having a broken CNTFRQ value that is hardly
ever used, especially since this affects quite a few systems.

Will