Re: [PATCH v5 18/18] qapi: introduce DEVICE_ON event

[Michael S. Tsirkin]

On Thu, Mar 02, 2023 at 11:39:42AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 02.03.23 00:07, Michael S. Tsirkin wrote:
> > On Thu, Feb 16, 2023 at 09:03:56PM +0300, Vladimir Sementsov-Ogievskiy 
> > wrote:
> > > We have DEVICE_DELETED event, that signals that device_del command is
> > > actually completed. But we don't have a counter-part for device_add.
> > > Still it's sensible for SHPC and PCIe-native hotplug, as there are time
> > > when the device in some intermediate state. Let's add an event that say
> > > that the device is finally powered on, power indicator is on and
> > > everything is OK for next manipulation on that device.
> > > 
> > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> > 
> > I don't much mind though a bit more motivation would be nice.
> > How is this going to be used? When does management care?
> 
> Some motivations:
> 
> 1. To be sure that device is "accepted" by guest. Guest may ignore hotplugged 
> device for some reason (for example during OS booting). Management wants to 
> catch this and handle the problem, instead of silent assume that everything 
> is OK. So, if we don't get the event by some timeout, we can report an error, 
> try to unplug/plug the disk again or do some other things to handle the 
> problem.

Note this is kind of weak in that we don't know that there's a driver.

> 2. The device can't be removed (by blockdev-del) while power indicator of 
> hotplug controller is blinking (QEMU reports "guest is busy (power indicator 
> blinking)"). So, management should avoid removing the device until it gets 
> the DEVICE_ON event.

That always annoyed me. I wanted delete to just stay pending until
it triggers.

But if we can't fix that,  it's a good reason.  However it can always
start blinking again. So DEVICE_ON is not a good name. DEVICE_REMOVABLE?
And not it's not realiable, it can start blinking by the time you try to
remove.
Another problem is that guest can create a flood of these events
by starting/stopping blinking.

Maybe, if blockdev-del fails then we start listening for events
and notify when it can be retried?

DEVICE_DELETED_RETRY ?

> 3. Also, management tool may make a GUI visualization of power indicator with 
> help of this event.
> 
> > 
> > Meanwhile, for the schema - can this one get ACKs from QAPI maintainers 
> > please?
> > 
> > 
> > > ---
> > >   qapi/qdev.json | 10 ++++++++++
> > >   hw/pci/pcie.c  | 14 ++++++++++++++
> > >   hw/pci/shpc.c  | 12 ++++++++++++
> > >   3 files changed, 36 insertions(+)
> > > 
> > > diff --git a/qapi/qdev.json b/qapi/qdev.json
> > > index 6f2d8d6647..116a8a7de8 100644
> > > --- a/qapi/qdev.json
> > > +++ b/qapi/qdev.json
> > > @@ -348,3 +348,13 @@
> > >   { 'command': 'query-hotplug',
> > >     'data': { 'id': 'str' },
> > >     'returns': 'HotplugInfo' }
> > > +
> > > +##
> > > +# @DEVICE_ON:
> > > +#
> > > +# Emitted whenever the device insertion completion is acknowledged by 
> > > the guest.
> > > +# For now only emitted for SHPC and PCIe-native hotplug.
> > > +#
> > > +# Since: 8.0
> > > +##
> > > +{ 'event': 'DEVICE_ON', 'data': 'DeviceAndPath' }
> > > diff --git a/hw/pci/pcie.c b/hw/pci/pcie.c
> > > index 636f962a23..4297e4e8dc 100644
> > > --- a/hw/pci/pcie.c
> > > +++ b/hw/pci/pcie.c
> > > @@ -22,6 +22,7 @@
> > >   #include "monitor/qdev.h"
> > >   #include "qapi/error.h"
> > > +#include "qapi/qapi-events-qdev.h"
> > >   #include "hw/pci/pci_bridge.h"
> > >   #include "hw/pci/pcie.h"
> > >   #include "hw/pci/msix.h"
> > > @@ -47,6 +48,13 @@ static bool pcie_sltctl_powered_off(uint16_t sltctl)
> > >           && (sltctl & PCI_EXP_SLTCTL_PIC) == PCI_EXP_SLTCTL_PWR_IND_OFF;
> > >   }
> > > +static bool pcie_sltctl_powered_on(uint16_t sltctl)
> > > +{
> > > +    return (sltctl & PCI_EXP_SLTCTL_PCC) == PCI_EXP_SLTCTL_PWR_ON &&
> > > +        (sltctl & PCI_EXP_SLTCTL_PIC) == PCI_EXP_SLTCTL_PWR_IND_ON &&
> > > +        (sltctl & PCI_EXP_SLTCTL_AIC) == PCI_EXP_SLTCTL_ATTN_IND_OFF;
> > > +}
> > > +
> > >   static LedActivity pcie_led_state_to_qapi(uint16_t value)
> > >   {
> > >       switch (value) {
> > > @@ -816,6 +824,12 @@ void pcie_cap_slot_write_config(PCIDevice *dev,
> > >           qdev_hotplug_state_event(DEVICE(dev), NULL, child_dev, 
> > > &changed_state);
> > >       }
> > > +    if ((sltsta & PCI_EXP_SLTSTA_PDS) && pcie_sltctl_powered_on(val) &&
> > > +        !pcie_sltctl_powered_on(old_slt_ctl) && child_dev)
> > > +    {
> > > +        qapi_event_send_device_on(child_dev->id, 
> > > child_dev->canonical_path);
> > > +    }
> > > +
> > >       /*
> > >        * If the slot is populated, power indicator is off and power
> > >        * controller is off, it is safe to detach the devices.
> > > diff --git a/hw/pci/shpc.c b/hw/pci/shpc.c
> > > index 6a4f93949d..380b2b83b3 100644
> > > --- a/hw/pci/shpc.c
> > > +++ b/hw/pci/shpc.c
> > > @@ -299,6 +299,12 @@ static bool shpc_slot_is_off(uint8_t state, uint8_t 
> > > power, uint8_t attn)
> > >       return state == SHPC_STATE_DISABLED && power == SHPC_LED_OFF;
> > >   }
> > > +static bool shpc_slot_is_on(uint8_t state, uint8_t power, uint8_t attn)
> > > +{
> > > +    return state == SHPC_STATE_ENABLED && power == SHPC_LED_ON &&
> > > +        attn == SHPC_LED_OFF;
> > > +}
> > > +
> > >   static void shpc_slot_command(PCIDevice *d, uint8_t target,
> > >                                 uint8_t state, uint8_t power, uint8_t 
> > > attn)
> > >   {
> > > @@ -366,6 +372,12 @@ static void shpc_slot_command(PCIDevice *d, uint8_t 
> > > target,
> > >               SHPC_SLOT_EVENT_MRL |
> > >               SHPC_SLOT_EVENT_PRESENCE;
> > >       }
> > > +
> > > +    if (!shpc_slot_is_on(old_state, old_power, old_attn) &&
> > > +        shpc_slot_is_on(state, power, attn) && child_dev)
> > > +    {
> > > +        qapi_event_send_device_on(child_dev->id, 
> > > child_dev->canonical_path);
> > > +    }
> > >   }
> > >   static void shpc_command(PCIDevice *d)
> > > -- 
> > > 2.34.1
> > 
> 
> -- 
> Best regards,
> Vladimir