Re: Alternative to arm64.nopauth cmdline for disabling Pointer Authentication

[Pavan Kondeti <pavan.kondeti@oss.qualcomm.com>]

On Thu, Dec 04, 2025 at 12:04:42PM +0000, Mark Rutland wrote:
> Hi Pavan, Marc,
> 
> On Thu, Dec 04, 2025 at 04:06:12PM +0530, Pavan Kondeti wrote:
> > On Thu, Dec 04, 2025 at 09:15:29AM +0000, Marc Zyngier wrote:
> > > On Thu, 04 Dec 2025 04:07:15 +0000,
> > > Pavan Kondeti <pavan.kondeti@oss.qualcomm.com> wrote:
> > > > Hi
> > > > 
> > > > The pointer authentication feature (PAuth) is only supported on
> > > > 0-3 CPUs but it is not supported on 4-7 CPUS on QCS8300.
> 
> > > On what grounds? Hardware incompatibility? I seriously doubt it,
> > > since nobody glues pre-8.3 CPUs to anything more modern.
> 
> Marc, it seems like that's exactly that's happened here. :/
> 
> > I see that Linux runs at EL2 and AA64ISAR1 register values on CPU#0
> > (A78) indicates that PAuth is supported but not for CPU#4 (A55). I am
> > told, there are no other controls outside EL2 (trap) to manipulate
> > this feature. So, I am assuming that this is indeed reflecting the HW.
> 
> Cortex-A78 doesn't have pointer authentication, but Cortex-A78C does, so
> maybe you actually have Cortex-A78C?
> 
> What are the MIDRs on this SoC? e.g. the output of:
> 
>   cat /sys/devices/system/cpu/cpu*/regs/identification/midr_el1 | sort | uniq
> 
> If this SoC has both Cortex-A78C and Cortex-A55, that's a rather
> unfortunate combination that should have been caught at the SoC design
> phase, as Marc alluded to.

Yes. The MIDR register indicate the same. A78C + A55

root@v8a-arm64:~# cat /sys/devices/system/cpu/cpu*/regs/identification/midr_el1
0x00000000410fd4b2
0x00000000410fd4b2
0x00000000410fd4b2
0x00000000410fd4b2
0x00000000412fd050
0x00000000412fd050
0x00000000412fd050
0x00000000412fd050

> 
> [...]
> 
> > > > This patch [2] from Catalin adds a devicetree property under memory {}
> > > > to disable MTE.
> 
> Catalin's patch was a bit different. It described whether the memory
> range supported tags (and was actually a property of the memory). That
> patch didn't make it into mainline AFAICT.
> 
> [...]
> 
> > I understand that this does not fall under errata but is
> > there a possiblity to introduce an Errata targeting CPU#0 MIDR and
> > disabling the Pointer authentication? I understand that if there is
> > another Qualcomm SoC that exists with all CPUs supporting pointer
> > authentication with same MIDR, we may be disabling the feature but this
> > is something I can check internally.
> 
> I strongly suspect we cannot use the MIDR for this. Per:
> 
>   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=53a52a0ec7680287b170b36488203b5822e6da2d
> 
> ... Qualcomm's Kryo {Prime,Gold,Silver} parts seem to identify
> themselves as Arm ltd implementations, and those will appear in other
> non-Qualcomm SoCs.

Thank you for explaining this. Agree that we can't use MIDR based
workaround here. 
> 
> Without know *exactly* which MIDRs you have on this part, we cannot say.
> 
> Regardless, I strongly suspect that we have to live with the command
> line option for these parts.
> 

Got it. 

Thanks,
Pavan