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SubjectRe: [PATCH v3 20/23] kvm: arm64: Intercept host's CPU_SUSPEND PSCI SMCs
On Thu, Nov 26, 2020 at 03:54:18PM +0000, David Brazdil wrote:
> Add a handler of CPU_SUSPEND host PSCI SMCs. The SMC can either enter
> a sleep state indistinguishable from a WFI or a deeper sleep state that
> behaves like a CPU_OFF+CPU_ON except that the core is still considered
> online when asleep.
>
> The handler saves r0,pc of the host and makes the same call to EL3 with
> the hyp CPU entry point. It either returns back to the handler and then
> back to the host, or wakes up into the entry point and initializes EL2
> state before dropping back to EL1.

For those CPU_SUSPEND calls which lose context, is there no EL2 state
that you need to save/restore, or is that all saved elsewhere already?

The usual suspects are PMU, debug, and timers, so maybe not. It'd be
nice to have a statement in the commit message if we're certain there's
no state that we need to save.

> A core can only suspend itself but other cores can concurrently invoke
> CPU_ON with this core as target. To avoid racing them for the same
> boot args struct, CPU_SUSPEND uses a different struct instance and entry
> point. Each entry point selects the corresponding struct to restore host
> boot args from. This avoids the need for locking in CPU_SUSPEND.

I found this a bit confusing since the first sentence can be read to
mean that CPU_ON is expected to compose with CPU_SUSPEND, whereas what
this is actually saying is the implementation ensures they don't
interact. How about:

| CPU_ON and CPU_SUSPEND are both implemented using struct cpu_boot_args
| to store the state upon powerup, with each CPU having separate structs
| for CPU_ON and CPU_SUSPEND so that CPU_SUSPEND can operate locklessly
| and so that a CPU_ON xall targetting a CPU cannot interfere with a
| concurrent CPU_SUSPEND call on that CPU.

The patch itself looks fine to me.

Thanks,
Mark.

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