Subject: [PATCH] ARM: KVM: iterate over all CPUs for CPU compatibility check

From: [email-masked] (Marc Zyngier)
Date: Fri, 12 Apr 2013 14:24:09 +0100

On 12/04/13 14:04, Andre Przywara wrote:
> kvm_target_cpus() checks the compatibility of the used CPU with
> KVM, which is currently limited to ARM Cortex-A15 cores.
> However by calling it only once on any random CPU it assumes that
> all cores are the same, which is not true for big.LITTLE parts.
> 
> After doing about 40 boots on a TC-2 core tile, I found it running
> in all but one case on one of the A7 cores (which correctly denied
> KVM initialization). On the 39th boot however the code ran on
> an A15, leading to a hang after returning success:
> 
> ...
> TCP: reno registered
> UDP hash table entries: 512 (order: 2, 16384 bytes)
> UDP-Lite hash table entries: 512 (order: 2, 16384 bytes)
> NET: Registered protocol family 1
> kvm_target_cpu() on CPU #1, part is c0f0
>  ... (pause for a while) ...
> INFO: rcu_sched self-detected stall on CPUINFO: rcu_sched detected stalls on CPU
> s/tasks: { 1} (detected by 0, t=6002 jiffies, g=4294966999, c=4294966998, q=15)
> Task dump for CPU 1:
> swapper/0       R running      0     1      0 0x00000002
> 
> So iterate over every CPU to correctly determine the capability of
> the system to run the current KVM implementation.
> In case a big.LITTLE configuration is the reason for denial, give
> the user a hint how to get it running anyway (maxcpus= on the kernel
> command line).
> 
> Please push this still into 3.9.
> 
> Signed-off-by: Andre Przywara <[email-masked]>

[...]

Nak. The fact that one of the CPUs seem to hang is a sure sign that
something is severely broken, and you definitely want to fix that issue,
instead of blindly ignoring it.

Additionally, it seems you're just papering over the issue. You should
be able to exclude the A7 processors, but not completely deny KVM from
running on the hardware.

	M.
-- 
Jazz is not dead. It just smells funny...