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From: Thomas Gleixner <tglx@linutronix.de>
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To: Sven Schnelle <svens@linux.ibm.com>, Peter Zijlstra
 <peterz@infradead.org>, Andy Lutomirski <luto@kernel.org>
Cc: linux-kernel@vger.kernel.org, Heiko Carstens <hca@linux.ibm.com>
Subject: Re: [PATCH 1/3] entry: move exit to usermode functions to header file
In-Reply-To: <20231205133015.752543-2-svens@linux.ibm.com>
References: <20231205133015.752543-1-svens@linux.ibm.com>
 <20231205133015.752543-2-svens@linux.ibm.com>
Date: Fri, 15 Dec 2023 20:09:27 +0100
Message-ID: <87r0jnmgew.ffs@tglx>
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On Tue, Dec 05 2023 at 14:30, Sven Schnelle wrote:
> +/**
> + * exit_to_user_mode_loop - do any pending work before leaving to user space
> + */
> +static __always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> +							    unsigned long ti_work)
> +{
> +	/*
> +	 * Before returning to user space ensure that all pending work
> +	 * items have been completed.
> +	 */
> +	while (ti_work & EXIT_TO_USER_MODE_WORK) {
> +
> +		local_irq_enable_exit_to_user(ti_work);
> +
> +		if (ti_work & _TIF_NEED_RESCHED)
> +			schedule();
> +
> +		if (ti_work & _TIF_UPROBE)
> +			uprobe_notify_resume(regs);
> +
> +		if (ti_work & _TIF_PATCH_PENDING)
> +			klp_update_patch_state(current);
> +
> +		if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
> +			arch_do_signal_or_restart(regs);
> +
> +		if (ti_work & _TIF_NOTIFY_RESUME)
> +			resume_user_mode_work(regs);
> +
> +		/* Architecture specific TIF work */
> +		arch_exit_to_user_mode_work(regs, ti_work);
> +
> +		/*
> +		 * Disable interrupts and reevaluate the work flags as they
> +		 * might have changed while interrupts and preemption was
> +		 * enabled above.
> +		 */
> +		local_irq_disable_exit_to_user();
> +
> +		/* Check if any of the above work has queued a deferred wakeup */
> +		tick_nohz_user_enter_prepare();
> +
> +		ti_work = read_thread_flags();
> +	}
> +
> +	/* Return the latest work state for arch_exit_to_user_mode() */
> +	return ti_work;
> +}

I'm not really sure about this part. exit_to_user_mode_loop() is the
slowpath when a TIF work flag is set. I can see the benefit on the
fastpath functions which are way smaller.

Thanks,

        tglx