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spapr: rollback 'unplug timeout' for CPU hotunplugs

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The pseries machines introduced the concept of 'unplug timeout' for CPU
hotunplugs. The idea was to circunvent a deficiency in the pSeries
specification (PAPR), that currently does not define a proper way for
the hotunplug to fail. If the guest refuses to release the CPU (see [1]
for an example) there is no way for QEMU to detect the failure.

Further discussions about how to send a QAPI event to inform about the
hotunplug timeout [2] exposed problems that weren't predicted back when
the idea was developed. Other QEMU machines don't have any type of
hotunplug timeout mechanism for any device, e.g. ACPI based machines
have a way to make hotunplug errors visible to the hypervisor. This
would make this timeout mechanism exclusive to pSeries, which is not
ideal.

The real problem is that a QAPI event that reports hotunplug timeouts
puts the management layer (namely Libvirt) in a weird spot. We're not
telling that the hotunplug failed, because we can't be 100% sure of
that, and yet we're resetting the unplug state back, preventing any
DEVICE_DEL events to reach out in case the guest decides to release the
device. Libvirt would need to inspect the guest itself to see if the
device was released or not, otherwise the internal domain states will be
inconsistent.  Moreover, Libvirt already has an 'unplug timeout'
concept, and a QEMU side timeout would need to be juggled together with
the existing Libvirt timeout.

All this considered, this solution ended up creating more trouble than
it solved. This patch reverts the 3 commits that introduced the timeout
mechanism for CPU hotplugs in pSeries machines.

This reverts commit 4515a5f786
"qemu_timer.c: add timer_deadline_ms() helper"

This reverts commit d1c2e3ce3d
"spapr_drc.c: add hotunplug timeout for CPUs"

This reverts commit 51254ffb32
"spapr_drc.c: introduce unplug_timeout_timer"

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1911414
[2] https://lists.gnu.org/archive/html/qemu-devel/2021-03/msg04682.html

CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20210401000437.131140-2-danielhb413@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
This commit is contained in:
Daniel Henrique Barboza 2021-03-31 21:04:36 -03:00 • committed by David Gibson
parent 555249a59e
commit d522cb52e6
5 changed files with 0 additions and 82 deletions
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13
util/qemu-timer.c
View file

@ -242,19 +242,6 @@ int64_t timerlist_deadline_ns(QEMUTimerList *timer_list)
return delta;
}
/*
* Returns the time remaining for the deadline, in ms.
*/
int64_t timer_deadline_ms(QEMUTimer *timer)
{
if (timer_pending(timer)) {
return qemu_timeout_ns_to_ms(timer->expire_time) -
qemu_clock_get_ms(timer->timer_list->clock->type);
}
return 0;
}
/* Calculate the soonest deadline across all timerlists attached
* to the clock. This is used for the icount timeout so we
* ignore whether or not the clock should be used in deadline