[PATCH 2/2] arm64/kvm: survive unknown traps from guests

[Suzuki K Poulose]

On 20/02/17 12:30, Mark Rutland wrote:
> Currently we BUG() if we see an ESR_EL2.EC value we don't recognise. As
> configurable disables/enables are added to the architecture (controlled
> by RES1/RES0 bits respectively), with associated synchronous exceptions,
> it may be possible for a guest to trigger exceptions with classes that
> we don't recognise.
>
> While we can't service these exceptions in a manner useful to the guest,
> we can avoid bringing down the host. Per ARM DDI 0487A.k_iss10775, page
> D7-1937, EC values within the range 0x00 - 0x2c are reserved for future
> use with synchronous exceptions, and EC values within the range 0x2d -
> 0x3f may be used for either synchronous or asynchronous exceptions.
>
> The patch makes KVM handle any unknown EC by injecting an UNDEFINED
> exception into the guest, with a corresponding (ratelimited) warning in
> the host dmesg. We could later improve on this with with a new (opt-in)
> exit to the host userspace.

If the unknown EC happens to be an asynchronous exception (allocated from the second range),
we are not sure if that was triggered by this guest, or a different guest or even
the hypervisor if we cannot really isolate the exception(with/without the RAS
extensions, i.e ESB). And I accept there may not be a perfect solution to the problem
either. May be we could explicitly mention about "unsure" exceptions, so that
it might help, people who may not really have the deep knowledge about the
exception code schemes.

Suzuki