[RFC PATCH v3 06/16] KVM: arm64: Introduce SPE primitives

[James Morse]

Hi Alex,

On 27/10/2020 17:26, Alexandru Elisei wrote:
> KVM SPE emulation depends on the configuration option KVM_ARM_SPE and on on
> having hardware SPE support on all CPUs.

> The host driver must be
> compiled-in because we need the SPE interrupt to be enabled; it will be
> used to kick us out of the guest when the profiling buffer management
> interrupt is asserted by the GIC (for example, when the buffer is full).

Great: SPE IRQ very important...


> Add a VCPU flag to inform KVM that the guest has SPE enabled.
> 
> It's worth noting that even though the KVM_ARM_SPE config option is gated
> by the SPE host driver being compiled-in, we don't actually check that the
> driver was loaded successfully when we advertise SPE support for guests.

Eh?

> That's because we can live with the SPE interrupt being disabled. There is
> a delay between when the SPE hardware asserts the interrupt and when the
> GIC samples the interrupt line and asserts it to the CPU. If the SPE
> interrupt is disabled at the GIC level, this delay will be larger,

How does this work? Surely the IRQ needs to be enabled before it can become pending at the
CPU to kick us out of the guest...


> at most a host timer tick.

(Because the timer brings us out of the guest anyway?)


Thanks,

James