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Re: [qemu-s390x] [PATCH v3] s390x/tod: Properly stop the KVM TOD while t

From: David Hildenbrand
Subject: Re: [qemu-s390x] [PATCH v3] s390x/tod: Properly stop the KVM TOD while the guest is not running
Date: Fri, 30 Nov 2018 13:47:09 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.0

On 30.11.18 13:11, Cornelia Huck wrote:
> On Fri, 30 Nov 2018 10:49:57 +0100
> David Hildenbrand <address@hidden> wrote:
>> Just like on other architectures, we should stop the clock while the guest
>> is not running. This is already properly done for TCG. Right now, doing an
>> offline migration (stop, migrate, cont) can easily trigger stalls in the
>> guest.
>> Even doing a
>>     (hmp) stop
>>     ... wait 2 minutes ...
>>     (hmp) cont
>> will already trigger stalls.
>> So whenever the guest stops, backup the KVM TOD. When continuing to run
>> the guest, restore the KVM TOD.
>> One special case is starting a simple VM: Reading the TOD from KVM to
>> stop it right away until the guest is actually started means that the
>> time of any simple VM will already differ to the host time. We can
>> simply leave the TOD running and the guest won't be able to recognize
>> it.
>> For migration, we actually want to keep the TOD stopped until really
>> starting the guest. To be able to catch most errors, we should however
>> try to set the TOD in addition to simply storing it. So we can still
>> catch basic migration problems.
>> If anything goes wrong while backing up/restoring the TOD, we have to
>> ignore it (but print a warning). This is then basically a fallback to
>> old behavior (TOD remains running).
>> I tested this very basically with an initrd:
>>     1. Start a simple VM. Observed that the TOD is kept running. Old
>>        behavior.
>>     2. Ordinary live migration. Observed that the TOD is temporarily
>>        stopped on the destination when setting the new value and
>>        correctly started when finally starting the guest.
>>     3. Offline live migration. (stop, migrate, cont). Observed that the
>>        TOD will be stopped on the source with the "stop" command. On the
>>        destination, the TOD is temporarily stopped when setting the new
>>        value and correctly started when finally starting the guest via
>>        "cont".
>>     4. Simple stop/cont correctly stops/starts the TOD. (multiple stops
>>        or conts in a row have no effect, so works as expected)
>> In the future, we might want to send the guest a special kind of time sync
>> interrupt under some conditions, so it can synchronize its tod to the
>> host tod. This is interesting for migration scenarios but also when we
>> get time sync interrupts ourselves. This however will most probably have
>> to be handled in KVM (e.g. when the tods differ too much) and is not
>> desired e.g. when debugging the guest. (single stepping should not
> I still plan to drop the extra '.' :)

If there is a v4, I promise it will be gone ;)

>>  hw/s390x/tod-kvm.c     | 102 ++++++++++++++++++++++++++++++++++++++++-
>>  include/hw/s390x/tod.h |   8 +++-
>>  2 files changed, 107 insertions(+), 3 deletions(-)
> Passes my smoke tests. Will queue after I get reviews.



David / dhildenb

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