Hi Marc,

On Wed, Aug 30, 2017 at 09:46:26AM +0100, Marc Zyngier wrote:

[...]

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This is completely busted:

- You are only addressing the kernel side, and ignore userspace (which
is just as broken as the kernel).

Thanks for quick reviewing.

Yeah, I remembered there have some code directly reads arch timer
virtual counter from userspace. But can you remind for userspace
broken issue, is the code in libc or vdso?

You're missing the point. The virtual counter is freely available to
userspace to play with (this is a de-facto ABI). Since it can return bad
values, it needs to be trapped to be correctly emulated (together with
cntfrq_el0), and the VDSO disabled.

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Here I have another question is: after applied the whole workaround
infrastructure, can it also fix userspace broken issue?

Yup. That's why I ended-up with a 18 patches series, and not just this
single one. Trust me, I'm lazy. There is nothing I hate more than doing
useless work.

Thanks for detailed explaination. Now it's much clear for me.

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- You lie in the description of the option (this is in no way dynamic,
since you didn't backport the whole workaround infrastructure).

I should fix it.

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So I'm afraid I'm NAKing this. Please refrain from blindly backporting
random patches. fa8d815fac96e7c9 only makes sense in the context of the
whole series, and on its own gives you a very false sense of having
properly addressed it.

IIUC, at least fa8d815fac96e7c9 can fix issue in kernel side, such like
for sched_clock() roll back issue [1].

What's the point of fixing the kernel if userspace is just as likely to
fail?

Understand now.

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So for this issue, are you suggesting we need backport whole workaround
infrastructure onto kernel 4.4 and 4.9? Many ARM devices are working
with these two kernels.

Then these systems are completely broken if they use a Cortex-A73.
Either they run mainline (which will be just fine), or they get a fully
backported workaround infrastructure.

Yeah, we should do right thing.

Have ARM kernel team ported this patch series (or is in planning)? I
also will check with Linaro kernel team as well, I just want to avoid
duplicate efforts if these patches have been back ported. Otherwise, I
will backport the patch series.

Thanks,
Leo Yan