linux-kernel - Re: [RFC PATCH v8 13/14] xpfo, mm: Defer TLB flushes for non-current CPUs (x86 only)

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Message-ID: <a6510fa8-e96d-677b-78df-da9a19c4089b@intel.com>
Date:   Thu, 14 Feb 2019 09:42:27 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Khalid Aziz <khalid.aziz@...cle.com>, juergh@...il.com,
        tycho@...ho.ws, jsteckli@...zon.de, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, liran.alon@...cle.com,
        keescook@...gle.com, akpm@...ux-foundation.org, mhocko@...e.com,
        catalin.marinas@....com, will.deacon@....com, jmorris@...ei.org,
        konrad.wilk@...cle.com
Cc:     deepa.srinivasan@...cle.com, chris.hyser@...cle.com,
        tyhicks@...onical.com, dwmw@...zon.co.uk,
        andrew.cooper3@...rix.com, jcm@...hat.com,
        boris.ostrovsky@...cle.com, kanth.ghatraju@...cle.com,
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        pradeep.vincent@...cle.com, john.haxby@...cle.com,
        tglx@...utronix.de, kirill.shutemov@...ux.intel.com, hch@....de,
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        peterz@...radead.org, kernel-hardening@...ts.openwall.com,
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        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v8 13/14] xpfo, mm: Defer TLB flushes for non-current
 CPUs (x86 only)

>  #endif
> +
> +	/* If there is a pending TLB flush for this CPU due to XPFO
> +	 * flush, do it now.
> +	 */

Don't forget CodingStyle in all this, please.

> +	if (cpumask_test_and_clear_cpu(cpu, &pending_xpfo_flush)) {
> +		count_vm_tlb_event(NR_TLB_REMOTE_FLUSH_RECEIVED);
> +		__flush_tlb_all();
> +	}

This seems to exist in parallel with all of the cpu_tlbstate
infrastructure.  Shouldn't it go in there?

Also, if we're doing full flushes like this, it seems a bit wasteful to
then go and do later things like invalidate_user_asid() when we *know*
that the asid would have been flushed by this operation.  I'm pretty
sure this isn't the only __flush_tlb_all() callsite that does this, so
it's not really criticism of this patch specifically.  It's more of a
structural issue.

> +void xpfo_flush_tlb_kernel_range(unsigned long start, unsigned long end)
> +{

This is a bit lightly commented.  Please give this some good
descriptions about the logic behind the implementation and the tradeoffs
that are in play.

This is doing a local flush, but deferring the flushes on all other
processors, right?  Can you explain the logic behind that in a comment
here, please?  This also has to be called with preemption disabled, right?

> +	struct cpumask tmp_mask;
> +
> +	/* Balance as user space task's flush, a bit conservative */
> +	if (end == TLB_FLUSH_ALL ||
> +	    (end - start) > tlb_single_page_flush_ceiling << PAGE_SHIFT) {
> +		do_flush_tlb_all(NULL);
> +	} else {
> +		struct flush_tlb_info info;
> +
> +		info.start = start;
> +		info.end = end;
> +		do_kernel_range_flush(&info);
> +	}
> +	cpumask_setall(&tmp_mask);
> +	cpumask_clear_cpu(smp_processor_id(), &tmp_mask);
> +	cpumask_or(&pending_xpfo_flush, &pending_xpfo_flush, &tmp_mask);
> +}

Fun.  cpumask_setall() is non-atomic while cpumask_clear_cpu() and
cpumask_or() *are* atomic.  The cpumask_clear_cpu() is operating on
thread-local storage and doesn't need to be atomic.  Please make it
__cpumask_clear_cpu().