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

[Will Deacon]

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