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

[Marc Zyngier]

On Tue, 16 Apr 2013 09:26:26 -0700, Christoffer Dall
<cdall at cs.columbia.edu> wrote:
> On Mon, Apr 15, 2013 at 6:48 AM, Will Deacon <will.deacon at arm.com>
wrote:
>> On Mon, Apr 15, 2013 at 02:13:55PM +0100, Andre Przywara wrote:
>>> On 04/15/2013 11:52 AM, Alexander Spyridakis wrote:
>>> > I've run on this problem before, while trying to run KVM guests on
A7
>>> > cores.
>>> >
>>> > For some reason the 3rd A7 hangs in arch/arm/kvm/init.S, on the
>>> > instruction that updates HSCTLR between the two isbs on
__do_hyp_init
>>> > (mcr p15, 4, r0, c1, c0, 0). If you boot the system with maxcpus=4
>>> > then
>>> > init_hyp_mode() will not hang on the A7 cluster. Other than that
from
>>> > my
>>> > limited testing KVM on A7 works on a usual linux guest. I also tried
>>> > to
>>> > only boot the 3rd A7 core to rule out any racing issues, but still
the
>>> > same behaviour applies.
>>>
>>> Could well be the same issue here. I chased it down till CPU 2 goes
into
>>> HYP mode to do the initialization.
>>> I am running with maxcpus=3 (this increases the likelyhood that
>>> kvm_target_cpu() runs on an A15), so CPU #2 is the only one A7.
>>> As the HYP mode exception table is empty except for the HVC trap, it
may
>>> be looping here. I am trying now to get the PC of the faulty
>>> instruction.
>>
>> Yes, it sounds like you're taking a recursive fault because the vectors
>> aren't installed yet. Is there any chance you can find out what value
>> you end
>> up writing (or trying to write) to the HSCTLR please?
>>
> Actually I'm a little confused, wasn't Andre seeing a halt on an A15
> cpu, not an A7? Or is the theory that an A7 locks up and the calling
> A15 hangs on the SMP call to cpu_init_hyp_mode, waiting for the A7 to
> complete?

Yes, A15 hanging, not A7. That's why I'm strongly opposed to this patch.
I'm pretty sure the A7s only have a side effect that triggers a kernel bug
on the A15 side. Before taking *any* patch around this, we should
understand the issue fully, and not start patching random stuff just
because Linus is going to tag 3.9.

        M.
-- 
Fast, cheap, reliable. Pick two.