[3/3] arm64: Add software workaround for Falkor erratum 1041

[Manoj Iyer]

On Thu, 9 Nov 2017, Manoj Iyer wrote:

>
> James,
>
> (sorry for top-posting)
>
> Applied patch 3 patches to Ubuntu Artful Kernel ( 4.13.0-16-generic )
>
> - Start 20 VMs one at a time
>
> In a loop:
> - Stop (virsh destroy) 20 VMs one at a time
> - Start (virsh start) 20 VMs one at a time.

Fixing some confusion I might have introduced in my prev email.

- Applied all 3 patches to Ubuntu Artful Kernel ( 4.13.0-16-generic )

- Created 20 VMs one at a time

In a loop:
- Stop (virsh destroy) 20 VMs one at a time
- Start (virsh start) 20 VMs one at a time.

>
> The system reset's itself after starting the last VM on the 1st loop 
> displaying the following:
>
> awrep6 login: [ 603.349141] ACPI CPPC: PCC check channel failed. Status=0
> [ 603.765101] ACPI CPPC: PCC check channel failed. Status=0
> [ 603.937389] ACPI CPPC: PCC check channel failed. Status=0
> [ 608.285495] ACPI CPPC: PCC check channel failed. Status=0
> [ 608.289481] ACPI CPPC: PCC check channel failed. Status=0
>
> SYS_DBG: Running SDI image (immediate mode)
> SYS_DBG: Ram Dump Init
> SYS_DBG: Failed to init SD card
> SYS_DBG: Resetting system!
>
> Followed by the following messages on system reboot:
> [ 6.616891] BERT: Error records from previous boot:
> [ 6.621655] [Hardware Error]: event severity: fatal
> [ 6.626516] [Hardware Error]: imprecise tstamp: 0000-00-00 00:00:00
> [ 6.632851] [Hardware Error]: Error 0, type: fatal
> [ 6.637713] [Hardware Error]: section type: unknown, 
> d2e2621c-f936-468d-0d84-15a4ed015c8b
> [ 6.646045] [Hardware Error]: section length: 0x238
> [ 6.651082] [Hardware Error]: 00000000: 72724502 5220726f 6f736165 6e55206e 
> .Error Reason Un
> [ 6.659761] [Hardware Error]: 00000010: 776f6e6b 0000006e 00000000 00000000 
> known...........
> [ 6.668442] [Hardware Error]: 00000020: 00000000 00000000 00000000 00000000 
> ................
> [ 6.677122] [Hardware Error]: 00000030: 00000000 00000000 00000000 00000000 
> ................
>
>
> On Thu, 9 Nov 2017, James Morse wrote:
>
>> Hi Manoj,
>> 
>> On 08/11/17 19:05, Manoj Iyer wrote:
>>> On Thu, 2 Nov 2017, Shanker Donthineni wrote:
>>>> The ARM architecture defines the memory locations that are permitted
>>>> to be accessed as the result of a speculative instruction fetch from
>>>> an exception level for which all stages of translation are disabled.
>>>> Specifically, the core is permitted to speculatively fetch from the
>>>> 4KB region containing the current program counter and next 4KB.
>>>> 
>>>> When translation is changed from enabled to disabled for the running
>>>> exception level (SCTLR_ELn[M] changed from a value of 1 to 0), the
>>>> Falkor core may errantly speculatively access memory locations outside
>>>> of the 4KB region permitted by the architecture. The errant memory
>>>> access may lead to one of the following unexpected behaviors.
>> 
>>> I applied the 3 patches to Ubuntu 4.13.0-16-generic (Artful) kernel and
>>> ran stress-ng cpu tests on QDF2400 server
>> 
>> [...]
>> 
>>> Where stress-ng would spawn N workers and test cpu offline/online, perform
>>> matrix operations, do rapid context switchs, and anonymous mmaps. Although
>>> I was not able to reproduce the erratum on the stock 4.13 kernel using the
>>> same test case, the patched kernel did not seem to introduce any
>>> regressions either. I ran the stress-ng tests for over 8hrs found the
>>> system to be stable.
>> 
>> 
>> Could you throw kexec and KVM into the mix? This issue only shows up when 
>> we
>> disable the MMU, which we almost never do.
>> 
>> For CPU offline/online we make the PSCI 'offline' call with the MMU 
>> enabled.
>> When the CPU comes back firmware has reset the EL2/EL1 SCTLR from a higher
>> exception level, so it won't hit this issue.
>> 
>> One place we do this is kexec, where we drop into purgatory with the MMU 
>> disabled.
>> 
>> The other is KVM unloading itself to return to the hyp stub. You can stress 
>> this
>> by starting and stopping a VM. When the number of VMs reaches 0 KVM should
>> unload via 'kvm_arch_hardware_disable()'.
>> 
>> 
>> Thanks,
>> 
>> James
>> 
>> 
>
> --
> ============================
> Manoj Iyer
> Ubuntu/Canonical
> ARM Servers - Cloud
> ============================
>
>

--
============================
Manoj Iyer
Ubuntu/Canonical
ARM Servers - Cloud
============================