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 12:09:51 +0100
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To: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, Will Deacon <will@kernel.org>, Dave P Martin <Dave.Martin@arm.com>, Vincenzo Frascino <vincenzo.frascino@arm.com>, Szabolcs Nagy <szabolcs.nagy@arm.com>, Andrey Konovalov <andreyknvl@google.com>, Peter Collingbourne <pcc@google.com>, Andrew Morton <akpm@linux-foundation.org>

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.

It's not about kernel or user thread here. kthread_use_mm() (just
use_mm() in older kernels) would set an mm on a kernel thread,
temporarily making it behave as a user one. Since the sctlr_tcf0 is per
thread, not per mm, we need to switch to the default TCF0 for kthreads
so that user accesses (if use_mm() is called) don't generate any tag
check faults. Note that switch_mm() does not touch TCF0.

If we did allow a global, per-mm TCF0 setting, such kthreads could only
handle synchronous faults and no SIGSEGV generated (as we do with
copy_{from,to}_user() for normal threads).

If we want to revisit per-thread vs per-mm TCF0 setting, now is the
time.

-- 
Catalin