From mboxrd@z Thu Jan  1 00:00:00 1970
From: Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v7 18/29] arm64: mte: Allow user control of the tag check
 mode via prctl()
Date: Wed, 22 Jul 2020 11:28:47 +0100
Message-ID: <20200722102845.GA27540@gaia>
References: <20200715170844.30064-1-catalin.marinas@arm.com>
 <20200715170844.30064-19-catalin.marinas@arm.com>
 <e9feb87e-41a8-17e6-eeba-4038da3bdde2@arm.com>
 <20200720170050.GJ30452@arm.com>
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To: Dave Martin <Dave.Martin@arm.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>, linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, Szabolcs Nagy <szabolcs.nagy@arm.com>, Andrey Konovalov <andreyknvl@google.com>, Peter Collingbourne <pcc@google.com>, linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, Vincenzo Frascino <vincenzo.frascino@arm.com>, Will Deacon <will@kernel.org>

On Mon, Jul 20, 2020 at 06:00:50PM +0100, Dave P Martin wrote:
> On Mon, Jul 20, 2020 at 04:30:35PM +0100, Kevin Brodsky wrote:
> > On 15/07/2020 18:08, Catalin Marinas wrote:
> > >+void mte_thread_switch(struct task_struct *next)
> > >+{
> > >+	if (!system_supports_mte())
> > >+		return;
> > >+
> > >+	/* avoid expensive SCTLR_EL1 accesses if no change */
> > >+	if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
> > 
> > I think this could be improved by checking whether `next` is a kernel
> > thread, in which case thread.sctlr_tcf0 is 0 but there is no point in
> > setting SCTLR_EL1.TCF0, since there should not be any access via TTBR0.
> 
> Out of interest, do we have a nice way of testing for a kernel thread
> now?
> 
> I remember fpsimd_thread_switch() used to check for task->mm, but we
> seem to have got rid of that at some point.  set_mm() can defeat this,
> and anyway the heavy lifting for FPSIMD is now deferred until returning
> to userspace.

I think a better way is (current->flags & PF_KTHREAD). kthread_use_mm()
indeed changes current->mm.

-- 
Catalin