Message-ID: <b4594318-9ebf-91ce-b845-3fac92ba6476@arm.com>
Date:   Fri, 1 Feb 2019 09:15:21 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Sun Ted <xulinsun@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jeremy Linton <jeremy.linton@....com>,
        Jeffrey Hugo <jhugo@...eaurora.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>
Subject: Re: [PATCH 00/10] GICv3 support for kexec/kdump on EFI systems

Hi Xulin,

On 01/02/2019 06:11, Sun Ted wrote:
> Hi Marc,
> 
> Marc Zyngier <marc.zyngier@....com <mailto:marc.zyngier@....com>> 于2018
> 年9月22日周六 上午4:03写道：
> 
>     The GICv3 architecture has the remarkable feature that once LPI tables
>     have been assigned to redistributors and that LPI delivery is enabled,
>     there is no guarantee that LPIs can be turned off (and most
>     implementations do not allow it), nor can it be reprogrammed to use
>     other tables.
> 
>     This is a bit of a problem for kexec, where the secondary kernel
>     completely looses track of the previous allocations. If the secondary
>     kernel doesn't allocate the tables exactly the same way, no LPIs will
>     be delivered by the GIC (which continues to use the old tables), and
>     memory previously allocated for the pending tables will be slowly
>     corrupted, one bit at a time.
> 
>     The workaround for this is based on a series[1] by Ard Biesheuvel,
>     which adds the required infrastructure for memory reservations to be
>     passed from one kernel to another using an EFI table.
> 
>     This infrastructure is then used to register the allocation of GIC
>     tables with EFI, and allow the GIC driver to safely reuse the existing
>     programming if it detects that the tables have been correctly
>     registered. On non-EFI systems, there is not much we can do.
> 
> 
> Sorry to turn this question out again.
> For others non-EFI systems, just as your said till now we did do
> anything, right?

We didn't do anything, because there is nothing we can do.

> I did see the kexec booting failure since re-setting
> the GICR_CTLR.EnableLPI from "1" to "0" unsuccessful.
> 
> The  below commit adds the judgement for disabling LPIs, and returned
> error. Caused "kexec" booting failure.

And I fully expected this. When I said "we don't do anything", I meant
"we don't do anything to make LPIs work".

> 
> 6eb486b66a (irqchip/gic-v3: Ensure GICR_CTLR.EnableLPI=0 is observed
> before enabling).
> 
> <snip patch>
>  int its_cpu_init(void)
>  {
>         if (!list_empty(&its_nodes)) {
> -               if (!gic_rdists_supports_plpis()) {
> -                       pr_info("CPU%d: LPIs not supported\n",
> smp_processor_id());
> -                       return -ENXIO;
> -               }
> +               int ret;
> +
> +               ret = redist_disable_lpis();
> +               if (ret)
> +                       return ret;
> +

And I maintain that this is the right thing to do. If LPIs are on and
the memory has not been reserved, it is then likely that this memory is
now being used by the kernel for something else. The system is thus
going to see single-bit corruption in some random places.

At that stage, your system is horribly unsafe, and I will not let it
boot under these conditions. If it was working before, that's because
you were lucky, and I place no faith in luck.

Now you have two alternatives:

- You switch to an EFI-based firmware. These days, even u-boot has an
EFI implementation. COnsider doing that if you can.

- If there is no EFI implementation for your SoC, you can pass the
"irqchip.gicv3_nolpi=1" option to the first kernel. This will keep LPI
disabled, and you'll be able to kexec a secondary kernel (and get
working LPIs there). This is what I do on my Chromebook.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

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