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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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