[PATCH 2/5] KVM: arm64: Work around GICv3 locally generated SErrors

[Joey Gouly]

Hi Marc,

On Fri, Sep 24, 2021 at 09:25:39AM +0100, Marc Zyngier wrote:
> The infamous M1 has a feature nobody else ever implemented,
> in the form of the "GIC locally generated SError interrupts",
> also known as SEIS for short.
> 
> These SErrors are generated when a guest does something that violates
> the GIC state machine. It would have been simpler to just *ignore*
> the damned thing, but that's not what this HW does. Oh well.
> 
> This part of of the architecture is also amazingly under-specified.
> There is a whole 10 lines that describe the feature in a spec that
> is 930 pages long, and some of these lines are factually wrong.
> Oh, and it is deprecated, so the insentive to clarify it is low.
> 
> Now, the spec says that this should be a *virtual* SError when
> HCR_EL2.AMO is set. As it turns out, that's not always the case
> on this CPU, and the SError sometimes fires on the host as a
> physical SError. Goodbye, cruel world. This clearly is a HW bug,
> and it means that a guest can easily take the host down, on demand.
> 
> Thankfully, we have seen systems that were just as broken in the
> past, and we have the perfect vaccine for it.
> 
> Apple M1, please meet the Cavium ThunderX workaround. All your
> GIC accesses will be trapped, sanitised, and emulated. Only the
> signalling aspect of the HW will be used. It won't be super speedy,
> but it will at least be safe. You're most welcome.
> 
> Given that this has only ever been seen on this single implementation,
> that the spec is unclear at best and that we cannot trust it to ever
> be implemented correctly, gate the workaround solely on ICH_VTR_EL2.SEIS
> being set.
> 
> Signed-off-by: Marc Zyngier <maz at kernel.org>

[...]

I reproduced this issue on my M1 by using kvmtool and EDKII [1], and
have confirmed that this fixes it.

Tested-by: Joey Gouly <joey.gouly at arm.com>

Thanks,
Joey

[1] It is fixed in EDKII now, but I reverted Ard's EDKII commit locally:
  a82bad9730178a1e3a67c9bfc83412b87a8ad734