[EXT] Re: [PATCH v4 03/13] task_isolation: userspace hard isolation from kernel

[Alex Belits]

On Thu, 2020-10-01 at 16:40 +0200, Frederic Weisbecker wrote:
> External Email
> 
> -------------------------------------------------------------------
> ---
> On Wed, Jul 22, 2020 at 02:49:49PM +0000, Alex Belits wrote:
> > +/**
> > + * task_isolation_kernel_enter() - clear low-level task isolation
> > flag
> > + *
> > + * This should be called immediately after entering kernel.
> > + */
> > +static inline void task_isolation_kernel_enter(void)
> > +{
> > +	unsigned long flags;
> > +
> > +	/*
> > +	 * This function runs on a CPU that ran isolated task.
> > +	 *
> > +	 * We don't want this CPU running code from the rest of kernel
> > +	 * until other CPUs know that it is no longer isolated.
> > +	 * When CPU is running isolated task until this point anything
> > +	 * that causes an interrupt on this CPU must end up calling
> > this
> > +	 * before touching the rest of kernel. That is, this function
> > or
> > +	 * fast_task_isolation_cpu_cleanup() or stop_isolation()
> > calling
> > +	 * it. If any interrupt, including scheduling timer, arrives,
> > it
> > +	 * will still end up here early after entering kernel.
> > +	 * From this point interrupts are disabled until all CPUs will
> > see
> > +	 * that this CPU is no longer running isolated task.
> > +	 *
> > +	 * See also fast_task_isolation_cpu_cleanup().
> > +	 */
> > +	smp_rmb();
> 
> I'm a bit confused what this read memory barrier is ordering. Also
> against
> what it pairs.

My bad, I have kept it after there were left no write accesses from
other CPUs.

> 
> > +	if((this_cpu_read(ll_isol_flags) & FLAG_LL_TASK_ISOLATION) ==
> > 0)
> > +		return;
> > +
> > +	local_irq_save(flags);
> > +
> > +	/* Clear low-level flags */
> > +	this_cpu_write(ll_isol_flags, 0);
> > +
> > +	/*
> > +	 * If something happened that requires a barrier that would
> > +	 * otherwise be called from remote CPUs by CPU kick procedure,
> > +	 * this barrier runs instead of it. After this barrier, CPU
> > +	 * kick procedure would see the updated ll_isol_flags, so it
> > +	 * will run its own IPI to trigger a barrier.
> > +	 */
> > +	smp_mb();
> > +	/*
> > +	 * Synchronize instructions -- this CPU was not kicked while
> > +	 * in isolated mode, so it might require synchronization.
> > +	 * There might be an IPI if kick procedure happened and
> > +	 * ll_isol_flags was already updated while it assembled a CPU
> > +	 * mask. However if this did not happen, synchronize everything
> > +	 * here.
> > +	 */
> > +	instr_sync();
> 
> It's the first time I meet an instruction barrier. I should get
> information
> about that but what is it ordering here?

Against barriers in instruction cache flushing (flush_icache_range()
and such). 

> > +	local_irq_restore(flags);
> > +}
> 
> Thanks.