linux-kernel - Re: [PATCH] arm64: Select POSIX_CPU_TIMERS_TASK

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Date:   Wed, 27 Oct 2021 13:20:23 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        catalin.marinas@....com, will@...nel.org, tglx@...utronix.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: Select POSIX_CPU_TIMERS_TASK_WORK

On Wed, 27 Oct 2021 12:26:58 +0100,
Mark Rutland <mark.rutland@....com> wrote:
> 
> [adding Marc Z to Cc, since this affects KVM]
> 
> On Mon, Oct 18, 2021 at 04:47:13PM +0200, Nicolas Saenz Julienne wrote:
> > With 6caa5812e2d1 ("KVM: arm64: Use generic KVM xfer to guest work
> > function") all arm64 exit paths are properly equipped to handle the
> > POSIX timers' task work.
> > 
> > Deferring timer callbacks to thread context, not only limits the amount
> > of time spent in hard interrupt context, but is a safer
> > implementation[1], and will allow PREEMPT_RT setups to use KVM[2].
> > 
> > So let's enable POSIX_CPU_TIMERS_TASK_WORK on arm64.
> > 
> > [1] https://lore.kernel.org/all/20200716201923.228696399@linutronix.de/
> > [2] https://www.spinics.net/lists/linux-rt-users/msg24860.html
> 
> Trivial nit: could we please make that second link:
> 
>   https://lore.kernel.org/linux-rt-users/87v92bdnlx.ffs@tglx/
> 
> > Signed-off-by: Nicolas Saenz Julienne <nsaenzju@...hat.com>
> 
> Regardless, this makes sense to me, and given you've tested it:
> 
> Acked-by: Mark Rutland <mark.rutland@....com>
> 
> Thanks,
> Mark.
> 
> > 
> > ---
> > 
> > This was tested by running all relevant kernel timer self-tests and
> > making sure KVM still works as expected.
> > 
> >  arch/arm64/Kconfig | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 789036cf74f5..ce0d0d254542 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -191,6 +191,7 @@ config ARM64
> >  	select HAVE_PERF_REGS
> >  	select HAVE_PERF_USER_STACK_DUMP
> >  	select HAVE_REGS_AND_STACK_ACCESS_API
> > +	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
> >  	select HAVE_FUNCTION_ARG_ACCESS_API
> >  	select HAVE_FUTEX_CMPXCHG if FUTEX
> >  	select MMU_GATHER_RCU_TABLE_FREE

Looks reasonable to me.

Acked-by: Marc Zyngier <maz@...nel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.