Re: [libvirt] [PATCH] libxl: support ACPI shutdown flag

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Apr 22, 2014 at 11:12:16AM -0600, Jim Fehlig wrote:
> Add support for VIR_DOMAIN_SHUTDOWN_ACPI_POWER_BTN flag in
> libxlDomainShutdownFlags().  Inspired by similar functionality
> in the Xen xl client.
> 
> Signed-off-by: Jim Fehlig <jfehlig@suse.com>
> ---
> 
> I considered invoking libxl_send_trigger() immediately when
> VIR_DOMAIN_SHUTDOWN_ACPI_POWER_BTN flag is specified, but in the
> end decided to only honor the flag if a "normal" shutdown request
> failed.  This behavior is similar to xl and conforms to the
> virDomainShutdownFlags() docs
> 
> "If @flags is set to zero, then the hypervisor will choose the method
> of shutdown it considers best. To have greater control pass one or
> more of the virDomainShutdownFlagValues. The order in which the
> hypervisor tries each shutdown method is undefined, and a hypervisor
> is not required to support all methods."
> 
> I'm certainly receptive to only invoking libxl_send_trigger() when
> VIR_DOMAIN_SHUTDOWN_ACPI_POWER_BTN is specified if folks think that
> is a better approach.

The way I consider shutdown to operate is slightly different to
how you've done it in your patch

 - flags == 0 -> arbitrary upto the hypervisor to decide.
   
   Here you invoke libxl_domain_shutdown() and raise error
   if this fails. This is a valid implementation, though
   I'd suggest you could choose to try libxl_send_trigger
   too in this scenario

   The QEMU driver considers flags == 0, to be equivalent to
   the union of all flags it supports. ie it effectivel changes
   flags==0 to be flags = (ACPI_POWER_BTN | GUEST_AGENT)

 - flags & VIR_DOMAIN_SHUTDOWN_NNNN -> exclusively try the
   choice specified by the user. 

   When VIR_DOMAIN_SHUTDOWN_ACPI_POWER_BTN is passed, you
   are still trying libxl_domain_shutdown() before you try
   to run libxl_send_trigger(). IMHO this is a bug - it
   should try libxl_send_trigger exclusively if that was
   the only bit that was specified in the flags.

IIUC  libxl_domain_shutdown() will use the Xen paravirt channel
to trigger a controlled shutdown in the guest. Currently we have
the following flags defined

    VIR_DOMAIN_SHUTDOWN_DEFAULT        = 0,        /* hypervisor choice */
    VIR_DOMAIN_SHUTDOWN_ACPI_POWER_BTN = (1 << 0), /* Send ACPI event */
    VIR_DOMAIN_SHUTDOWN_GUEST_AGENT    = (1 << 1), /* Use guest agent */
    VIR_DOMAIN_SHUTDOWN_INITCTL        = (1 << 2), /* Use initctl */
    VIR_DOMAIN_SHUTDOWN_SIGNAL         = (1 << 3), /* Send a signal */

None of those really map to the Xen paravirt shutdown
so I think we should define a new flag in the API.

    VIR_DOMAIN_SHUTDOWN_PARAVIRT = (1 << 4), /* Use paravirt guest control */

Then I think the pseudo code makes sense

  - If flags == 0
       flags = (PARAVIRT | ACPI_POWER_BTN)

  - If flags & PARAVIRT
       libxl_domain_shutdown()
       if (ret == 0)
          goto done
       if (ret != ERROR_NOPARAVIT) {
          virRaiseError(...)
          goto cleanup;
       }

  - If flags & ACPI_POWER_BTN
       libxl_send_trigger()
       if (ret == 0)
          goto done
       virRaiseError(...)
       goto cleanup;


This gives users/apps full control over which methods are
tried. In the flags==0 case it'll try as many as possible.
If neccessary the user can force it to only do ACPI power,
or only do paravirt.

Regards,
Daniel
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