linux-kernel - Re: [PATCH v2 1/2] arm64: dts: broadcom: bcmbca: bcm4908: Reserve CFE stub area

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Message-ID: <CAH5Ym4jgadgT=7NDULOZt4VaeoP8TFDcgxCsTdDcA0+AM-H=fw@mail.gmail.com>
Date: Thu, 10 Oct 2024 17:05:25 -0700
From: Sam Edwards <cfsworks@...il.com>
To: Florian Fainelli <florian.fainelli@...adcom.com>
Cc: Rafał Miłecki <rafal@...ecki.pl>, 
	William Zhang <william.zhang@...adcom.com>, Anand Gore <anand.gore@...adcom.com>, 
	Kursad Oney <kursad.oney@...adcom.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>, devicetree@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] arm64: dts: broadcom: bcmbca: bcm4908: Reserve CFE
 stub area

On Thu, Oct 10, 2024 at 3:57 PM Florian Fainelli
<florian.fainelli@...adcom.com> wrote:
>
> On 10/4/24 22:01, Sam Edwards wrote:
> > The CFE bootloader places a stub program in the first page of physical
> > memory to hold the secondary CPUs until the boot CPU writes the release
> > address, but does not splice a /reserved-memory node into the FDT to
> > protect it. If Linux overwrites this program before execution reaches
> > smp_prepare_cpus(), the secondary CPUs may become inaccessible.
> >
> > This is only a problem with CFE, and then only until the secondary CPUs
> > are brought online. Ideally, there would be some hypothetical mechanism
> > we could use to indicate that this area of memory is sensitive only
> > during boot. But as there is none, and since it is such a small amount
> > of memory, it is easiest to reserve it unconditionally.
>
> If we supported CPU hotplug on those platforms (do we?) then it actually
> does matter that this memory remains protected, and it cannot be
> reclaimed. This does not invalidate the commit message and I will take
> it as-is, but it it is not memory that we can necessarily reclaim that
> easily, if we did things properly.

I am looking at only one build of CFE, so don't take what I'm saying
as gospel, but as I understand it:

CFE implements only the spin-table method, which isn't dynamic. Once
the kernel calls for the secondary CPUs to be released, it cannot shut
them down and bring them up again, so they have no need to reenter the
spin stub.

With U-Boot+ATF, it's of course a different story: ATF needs to stay
resident in memory to implement PSCI (which _is_ dynamic), but U-Boot
inserts/overwrites the necessary DT structures to protect ATF and tell
Linux to use PSCI, so we don't need to be thinking about that here.

But, again, that's just my understanding. If there's a version of CFE
out there that loads ATF/similar or implements PSCI itself, then,
well, that'd invalidate this patch('s commit message). I doubt such a
variant of CFE exists, but you're much better equipped to confirm or
refute that possibility, given your familiarity with the platform and
internal details.

Best wishes,
Sam

>
> >
> > Therefore, add a /reserved-memory node to bcm4908.dtsi to protect the
> > first 4KiB of physical memory.
> >
> > Signed-off-by: Sam Edwards <CFSworks@...il.com>
> > ---
> >   arch/arm64/boot/dts/broadcom/bcmbca/bcm4908.dtsi | 10 ++++++++++
> >   1 file changed, 10 insertions(+)
> >
> > diff --git a/arch/arm64/boot/dts/broadcom/bcmbca/bcm4908.dtsi b/arch/arm64/boot/dts/broadcom/bcmbca/bcm4908.dtsi
> > index 8b924812322c..c51b92387fad 100644
> > --- a/arch/arm64/boot/dts/broadcom/bcmbca/bcm4908.dtsi
> > +++ b/arch/arm64/boot/dts/broadcom/bcmbca/bcm4908.dtsi
> > @@ -68,6 +68,16 @@ l2: l2-cache0 {
> >               };
> >       };
> >
> > +     reserved-memory {
> > +             #address-cells = <2>;
> > +             #size-cells = <2>;
> > +             ranges;
> > +
> > +             cfe-stub@0 {
> > +                     reg = <0x0 0x0 0x0 0x1000>;
> > +             };
> > +     };
> > +
> >       axi@...00000 {
> >               compatible = "simple-bus";
> >               #address-cells = <1>;
>
>
> --
> Florian