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

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Message-ID: <20211027112658.GA54628@C02TD0UTHF1T.local>
Date:   Wed, 27 Oct 2021 12:26:58 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     catalin.marinas@....com, will@...nel.org, tglx@...utronix.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        maz@...nel.org
Subject: Re: [PATCH] arm64: Select POSIX_CPU_TIMERS_TASK_WORK

[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
> -- 
> 2.32.0
>