linux-kernel - Re: [PATCH v13 6/7] arm64: mte: Report async tag faults before suspend

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Date:   Fri, 12 Feb 2021 13:19:44 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:     Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH v13 6/7] arm64: mte: Report async tag faults before
 suspend

On Fri, Feb 12, 2021 at 12:00:15PM +0000, Lorenzo Pieralisi wrote:
> On Thu, Feb 11, 2021 at 03:33:52PM +0000, Vincenzo Frascino wrote:
> > +void mte_suspend_enter(void)
> > +{
> > +	if (!system_supports_mte())
> > +		return;
> > +
> > +	/*
> > +	 * The barriers are required to guarantee that the indirect writes
> > +	 * to TFSR_EL1 are synchronized before we report the state.
> > +	 */
> > +	dsb(nsh);
> > +	isb();
> > +
> > +	/* Report SYS_TFSR_EL1 before suspend entry */
> > +	mte_check_tfsr_el1();
> > +}
> > +
> >  void mte_suspend_exit(void)
> >  {
> >  	if (!system_supports_mte())
> >  		return;
> >  
> >  	update_gcr_el1_excl(gcr_kernel_excl);
> > +
> > +	/* Clear SYS_TFSR_EL1 after suspend exit */
> > +	write_sysreg_s(0, SYS_TFSR_EL1);
> 
> AFAICS it is not needed, it is done already in __cpu_setup() (that is
> called by cpu_resume on return from cpu_suspend() from firmware).
> 
> However, I have a question. We are relying on context switch to set
> sctlr_el1_tfc0 right ? If that's the case, till the thread resuming from
> low power switches context we are running with SCTLR_EL1_TCF0 not
> reflecting the actual value.

I think you have a point here, though not for SCTLR_EL1 as it is already
restored. GCR_EL1 is only updated after some C code has run and may mess
up stack tagging when/if we ever support it. Anyway, something to worry
about later, I think even the boot path gets this wrong.

-- 
Catalin