linux-kernel - Re: [PATCH] irqchip: gic-v3-its: avoid truncating memory addresses

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Message-Id: <e090dcf7-b55c-4bbe-bfba-482cf318463e@app.fastmail.com>
Date: Tue, 20 Jan 2026 10:52:53 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Marc Zyngier" <maz@...nel.org>, "Arnd Bergmann" <arnd@...nel.org>
Cc: "Thomas Gleixner" <tglx@...utronix.de>,
 "Lorenzo Pieralisi" <lpieralisi@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] irqchip: gic-v3-its: avoid truncating memory addresses

On Tue, Jan 20, 2026, at 09:51, Marc Zyngier wrote:
> On Mon, 19 Jan 2026 20:15:12 +0000, Arnd Bergmann <arnd@...nel.org> wrote:
>> 
>> Change the itt_addr variable to the correct phys_addr_t type instead,
>> along with all other variables in this driver that hold a physical
>> address.
>
> Ah, nice catch. It's amazing that we've managed for so long with such
> a glaring bug. I guess most 32bit VMs don't have much memory above the
> 4GB limit.

Right, the upstream qemu model specifically has no lowmem above
the limit, as its RAM starts as 0x40000000 and the most lowmem we
support (with CONFIG_VMSPLIT_1G) is 3.75GiB, ending at 0xEFFFFFFF
physical.

There are some SoCs that start physical RAM higher up, but I suppose
these were either never tested with CONFIG_VMSPLIT_1G, or they don't
have a GICv3.

The patch I'm testing with allows lowmem to be at physically
discontinuous addresses, so this can happen on any SoC that has
its second DRAM controller mapped to a high address, potentially
even with the regular VMSPLIT_3G or VMSPLIT_2G.

>> I checked the gicv5 driver for the same problem, and it correctly
>> uses u64 variables,
>
> GICv5 is strictly 64bit, and cannot be plugged on a system that
> supports AArch32.

Right, makes sense.

   Arnd