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

[Manoj Iyer]

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.

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
>
>

--
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Manoj Iyer
Ubuntu/Canonical
ARM Servers - Cloud
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