[PATCH] RFC: ARM: do not handle Spectre V2 on Vexpress CA9

[linux@armlinux.org.uk (Russell King - ARM Linux)]

On Thu, Nov 01, 2018 at 01:56:41PM +0000, Robin Murphy wrote:
> Hi Linus,
> 
> On 01/11/2018 12:54, Linus Walleij wrote:
> >Since the introduction of the Spectre V2 fixes for ARMv7,
> >especially the BPIALL workaround, the Versatile Express CA9
> >with its fragile CA9 core tile simply doesn't boot for me
> >anymore.
> >
> >If I turn on low level debugging the boot log stops short
> >at:
> >
> >smp: Bringing up secondary CPUs ...
> >GIC: PPI13 is secure or misconfigured
> >CPU1: thread -1, cpu 1, socket 0, mpidr 80000001
> >CPU1: Spectre v2: using BPIALL workaround
> >GIC: PPI13 is secure or misconfigured
> >CPU1: thread -1, cpu 1, socket 0, mpidr 80000001
> >CPU1: Spectre v2: using BPIALL workaround
> >
> >This is pretty much consistent behaviour, I think it managed
> >to boot at one point but these fixes are definately rubbing
> >this CPU the wrong way.
> >
> >This (not elegant) workaround tries to work around it by
> >simply not applying any Spectre v2 fixes on the ARM
> >Vexpress CA9. My other A9 platforms seem to work fine
> >with the fixes, so this appears to be related to the
> >fragility of the core tile on this one reference design,
> >so maybe it would be acceptable to mitigate it like this.
> >
> >I don't know how much this could be related to my particular
> >specimen, it would be great if others with this machine
> >could verify the problem.
> 
> As far as I remember, the usual V2P_CA9 problem where the L2 cache locks up
> for unfathomable reasons can come and go reasonably consistently depending
> on the exact kernel configuration/version, and ultimately seems like it
> might be more sensitive to the general shape and timing of the early boot
> code than any particular feature. As such I'd be wary that there might still
> be some other combinations of options which would manage to boot with the
> mitigation left enabled, and others which won't even if (or at worst
> *because*) we do skip it.

There are - for the Versatile Express CT9x4 in my build/boot system,
it's normal for the specific config for that system to pass, but it
fails when combined with other platforms - merely due to a different
kernel memory layout.

It's also kernel version dependent - there's been versions where
neither configuration has booted.

The platform is basically unreliable - it's not called a "test chip"
for no reason!

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