Re: [PATCH v8 0/4] pci hotplug tracking

[Michael S. Tsirkin]

On Thu, Nov 02, 2023 at 03:00:01PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 02.11.23 14:31, Michael S. Tsirkin wrote:
> > On Thu, Oct 05, 2023 at 12:29:22PM +0300, Vladimir Sementsov-Ogievskiy 
> > wrote:
> > > Hi all!
> > > 
> > > Main thing this series does is DEVICE_ON event - a counter-part to
> > > DEVICE_DELETED. A guest-driven event that device is powered-on.
> > > Details are in patch 2. The new event is paried with corresponding
> > > command query-hotplug.
> > 
> > Several things questionable here:
> > 1. depending on guest activity you can get as many
> >     DEVICE_ON events as you like
> 
> No, I've made it so it may be sent only once per device

Maybe document that?

> > 2. it's just for shpc and native pcie - things are
> >     confusing enough for management, we should make sure
> >     it can work for all devices
> 
> Agree, I'm thinking about it
> 
> > 3. what about non hotpluggable devices? do we want the event for them?
> > 
> 
> I think, yes, especially if we make async=true|false flag for device_add, so 
> that successful device_add must be always followed by DEVICE_ON - like 
> device_del is followed by DEVICE_DELETED.
> 
> Maybe, to generalize, it should be called not DEVICE_ON (which mostly relate 
> to hotplug controller statuses) but DEVICE_ADDED - a full counterpart for 
> DEVICE_DELETED.
> 
> > 
> > I feel this needs actual motivation so we can judge what's the
> > right way to do it.
> 
> My first motivation for this series was the fact that successful device_add 
> doesn't guarantee that hard disk successfully hotplugged to the guest. It 
> relates to some problems with shpc/pcie hotplug we had in the past, and they 
> are mostly fixed. But still, for management tool it's good to understand that 
> all actions related to hotplug controller are done and we have "green light".

what does "successfully" mean though? E.g. a bunch of guests will not
properly show you the device if the disk is not formatted properly.

> 
> Recently new motivation come, as I described in my "ping" letter 
> <6bd19a07-5224-464d-b54d-1d738f5ba8f7@yandex-team.ru>, that we have a 
> performance degradation because of 7bed89958bfbf40df, which introduces 
> drain_call_rcu() in device_add, to make it more synchronous. So, my 
> suggestion is make it instead more asynchronous (probably with special flag) 
> and rely on DEVICE_ON event.

This one?

commit 7bed89958bfbf40df9ca681cefbdca63abdde39d
Author: Maxim Levitsky <mlevitsk@redhat.com>
Date:   Tue Oct 6 14:38:58 2020 +0200

    device_core: use drain_call_rcu in in qmp_device_add
    
    Soon, a device removal might only happen on RCU callback execution.
    This is okay for device-del which provides a DEVICE_DELETED event,
    but not for the failure case of device-add.  To avoid changing
    monitor semantics, just drain all pending RCU callbacks on error.
    
    Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
    Suggested-by: Stefan Hajnoczi <stefanha@gmail.com>
    Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
    Message-Id: <20200913160259.32145-4-mlevitsk@redhat.com>
    [Don't use it in qmp_device_del. - Paolo]
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
index e9b7228480..bcfb90a08f 100644
--- a/softmmu/qdev-monitor.c
+++ b/softmmu/qdev-monitor.c
@@ -803,6 +803,18 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, 
Error **errp)
         return;
     }
     dev = qdev_device_add(opts, errp);
+
+    /*
+     * Drain all pending RCU callbacks. This is done because
+     * some bus related operations can delay a device removal
+     * (in this case this can happen if device is added and then
+     * removed due to a configuration error)
+     * to a RCU callback, but user might expect that this interface
+     * will finish its job completely once qmp command returns result
+     * to the user
+     */
+    drain_call_rcu();
+
     if (!dev) {
         qemu_opts_del(opts);
         return;



So maybe just move drain_call_rcu under if (!dev) then and be done with
it?

-- 
MST