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    SubjectRe: [RFC PATCH 01/17] shm-signal: shared-memory signals
    Avi Kivity wrote:
    > Gregory Haskins wrote:
    >>>> +struct shm_signal_irq {
    >>>> + __u8 enabled;
    >>>> + __u8 pending;
    >>>> + __u8 dirty;
    >>>> +};
    >>> Some ABIs may choose to pad this, suggest explicit padding.
    >> Yeah, good idea. What is the official way to do this these days? Are
    >> GCC pragmas allowed?
    > I just add a __u8 pad[5] in such cases.

    Oh, duh. Dumb question. I was getting confused with "pack", not pad. :)

    >>>> +
    >>>> +struct shm_signal;
    >>>> +
    >>>> +struct shm_signal_ops {
    >>>> + int (*inject)(struct shm_signal *s);
    >>>> + void (*fault)(struct shm_signal *s, const char *fmt, ...);
    >>> Eww. Must we involve strings and printf formats?
    >> This is still somewhat of a immature part of the design. Its supposed
    >> to be used so that by default, its a panic. But on the host side, we
    >> can do something like inject a machine-check. That way malicious/broken
    >> guests cannot (should not? ;) be able to take down the host. Note today
    >> I do not map this to anything other than the default panic, so this
    >> needs some love.
    >> But given the asynchronous nature of the fault, I want to be sure we
    >> have decent accounting to avoid bug reports like "silent MCE kills the
    >> guest" ;) At least this way, we can log the fault string somewhere to
    >> get a clue.
    > I see.
    > This raises a point I've been thinking of - the symmetrical nature of
    > the API vs the assymetrical nature of guest/host or user/kernel
    > interfaces. This is most pronounced in ->inject(); in the host->guest
    > direction this is async (host can continue processing while the guest
    > is handling the interrupt), whereas in the guest->host direction it is
    > synchronous (the guest is blocked while the host is processing the
    > call, unless the host explicitly hands off work to a different thread).

    Note that this is exactly what I do (though it is device specific).
    venet-tap has a ioq_notifier registered on its "rx" ring (which is the
    tx-ring for the guest) that simply calls ioq_notify_disable() (which
    calls shm_signal_disable() under the covers) and it wakes its
    rx-thread. This all happens in the context of the hypercall, which then
    returns and allows the vcpu to re-enter guest mode immediately.


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