Re: [PATCH V1 00/32] Live Update

[Steven Sistare]

On 7/30/2020 12:52 PM, Daniel P. Berrangé wrote:
> On Thu, Jul 30, 2020 at 08:14:04AM -0700, Steve Sistare wrote:
>> Improve and extend the qemu functions that save and restore VM state so a
>> guest may be suspended and resumed with minimal pause time.  qemu may be
>> updated to a new version in between.
>>
>> The first set of patches adds the cprsave and cprload commands to save and
>> restore VM state, and allow the host kernel to be updated and rebooted in
>> between.  The VM must create guest RAM in a persistent shared memory file,
>> such as /dev/dax0.0 or persistant /dev/shm PKRAM as proposed in 
>> https://lore.kernel.org/lkml/1588812129-8596-1-git-send-email-anthony.yznaga@oracle.com/
>>
>> cprsave stops the VCPUs and saves VM device state in a simple file, and
>> thus supports any type of guest image and block device.  The caller must
>> not modify the VM's block devices between cprsave and cprload.
>>
>> cprsave and cprload support guests with vfio devices if the caller first
>> suspends the guest by issuing guest-suspend-ram to the qemu guest agent.
>> The guest drivers suspend methods flush outstanding requests and re-
>> initialize the devices, and thus there is no device state to save and
>> restore.
>>
>>    1 savevm: add vmstate handler iterators
>>    2 savevm: VM handlers mode mask
>>    3 savevm: QMP command for cprsave
>>    4 savevm: HMP Command for cprsave
>>    5 savevm: QMP command for cprload
>>    6 savevm: HMP Command for cprload
>>    7 savevm: QMP command for cprinfo
>>    8 savevm: HMP command for cprinfo
>>    9 savevm: prevent cprsave if memory is volatile
>>   10 kvmclock: restore paused KVM clock
>>   11 cpu: disable ticks when suspended
>>   12 vl: pause option
>>   13 gdbstub: gdb support for suspended state
>>
>> The next patches add a restart method that eliminates the persistent memory
>> constraint, and allows qemu to be updated across the restart, but does not
>> allow host reboot.  Anonymous memory segments used by the guest are
>> preserved across a re-exec of qemu, mapped at the same VA, via a proposed
>> madvise(MADV_DOEXEC) option in the Linux kernel.  See
>> https://lore.kernel.org/lkml/1595869887-23307-1-git-send-email-anthony.yznaga@oracle.com/
>>
>>   14 savevm: VMS_RESTART and cprsave restart
>>   15 vl: QEMU_START_FREEZE env var
>>   16 oslib: add qemu_clr_cloexec
>>   17 util: env var helpers
>>   18 osdep: import MADV_DOEXEC
>>   19 memory: ram_block_add cosmetic changes
>>   20 vl: add helper to request re-exec
>>   21 exec, memory: exec(3) to restart
>>   22 char: qio_channel_socket_accept reuse fd
>>   23 char: save/restore chardev socket fds
>>   24 ui: save/restore vnc socket fds
>>   25 char: save/restore chardev pty fds
> 
> Keeping FDs open across re-exec is a nice trick, but how are you dealing
> with the state associated with them, most especially the TLS encryption
> state ? AFAIK, there's no way to serialize/deserialize the TLS state that
> GNUTLS maintains, and the patches don't show any sign of dealing with
> this. IOW it looks like while the FD will be preserved, any TLS session
> running on it will fail.

I had not considered TLS.  If a non-qemu library maintains connection state, 
then
we won't be able to support it for live update until the library provides 
interfaces
to serialize the state.

For qemu objects, so far vmstate has been adequate to represent the devices with
descriptors that we preserve.

> I'm going to presume that you're probably just considering the TLS features
> out of scope for your patch series.  It would be useful if you have any
> info about this and other things you've considered out of scope for this
> patch series.

The descriptors covered in these patches are needed for our use case.  I realize
there are others that could perhaps be preserved, but we have not tried them.
Those descriptors are closed on exec as usual, and are reopened after exec. I
expect that we or others will support more over time.

> I'm not seeing anything in the block layer about preserving open FDs, so
> I presume you're just letting the block layer close and then re-open any
> FDs it has ?  

Correct.

> This would have the side effect that any locks held on the
> FDs are lost, so there's a potential race condition where another process
> could acquire the lock and prevent the re-exec completing. That said this
> is unavoidable, because Linux kernel is completely broken wrt keeping
> fnctl() locks held across a re-exec, always throwing away the locks if
> more than 1 thread is running [1].

Ouch.

- Steve

> 
> Regards,
> Daniel
> 
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1552621
>