linux-kernel - Re: [RFC PATCH] arch: ARM64: add isb before enable pan

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Date:   Fri, 8 Oct 2021 09:45:18 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Quentin Perret <qperret@...gle.com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ke Wang <ke.wang@...soc.com>, ping.zhou1@...soc.com
Subject: Re: [RFC PATCH] arch: ARM64: add isb before enable pan

On Fri, Oct 08, 2021 at 04:34:12PM +0800, Zhaoyang Huang wrote:
> On Fri, Oct 8, 2021 at 4:01 PM Will Deacon <will@...nel.org> wrote:
> > On Fri, Oct 08, 2021 at 02:07:49PM +0800, Huangzhaoyang wrote:
> > > From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > >
> > > set_pstate_pan failure is observed in an ARM64 system occasionaly on a reboot
> > > test, which can be work around by a msleep on the sw context. We assume
> > > suspicious on disorder of previous instr of disabling SW_PAN and add an isb here.
> > >
> > > PS:
> > > The bootup test failed with a invalid TTBR1_EL1 that equals 0x34000000, which is
> > > alike racing between on chip PAN and SW_PAN.
> >
> > Sorry, but I'm struggling to understand the problem here. Please could you
> > explain it in more detail?
> >
> >   - Why does a TTBR1_EL1 value of `0x34000000` indicate a race?
> >   - Can you explain the race that you think might be occurring?
> >   - Why does an ISB prevent the race?
> Please find panic logs[1], related codes[2], sample of debug patch[3]
> below. TTBR1_EL1 equals 0x34000000 when panic and can NOT be captured
> by the debug patch during retest (all entrances that msr ttbr1_el1 are
> under watch) which should work. Adding ISB here to prevent race on
> TTBR1 from previous access of sysregs which can affect the msr
> result(the test is still ongoing). Could the race be
> ARM64_HAS_PAN(automated by core) and SW_PAN.

Can you please change the ARM64_HAS_PAN type to
ARM64_CPUCAP_STRICT_BOOT_CPU_FEATURE? I wonder whether
system_uses_ttbr0_pan() changes its output when all CPUs had been
brought up and system_uses_hw_pan() returns true.

-- 
Catalin