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Re: [question] Shall we flush ITS tables into guest RAM when shutdown th

From: Kunkun Jiang
Subject: Re: [question] Shall we flush ITS tables into guest RAM when shutdown the VM?
Date: Mon, 26 Jul 2021 21:19:28 +0800
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 2021/7/6 22:27, Eric Auger wrote:
Hi Dave,

On 7/6/21 4:19 PM, Dr. David Alan Gilbert wrote:
* Eric Auger (eric.auger@redhat.com) wrote:

On 7/6/21 10:18 AM, Kunkun Jiang wrote:
Hi Eric,

On 2021/6/30 17:16, Eric Auger wrote:
On 6/30/21 3:38 AM, Kunkun Jiang wrote:
On 2021/6/30 4:14, Eric Auger wrote:
Hi Kunkun,

On 6/29/21 11:33 AM, Kunkun Jiang wrote:
Hi all,

Accroding to the patch cddafd8f353d2d251b1a5c6c948a577a85838582,
our original intention is to flush the ITS tables into guest RAM at
the point
RUN_STATE_FINISH_MIGRATE, but sometimes the VM gets stopped before
migration launch so let's simply flush the tables each time the VM

But I encountered an error when I shut down the virtual machine.

qemu-system-aarch64: KVM_SET_DEVICE_ATTR failed: Group 4 attr
0x0000000000000001: Permission denied
Shall we need to flush ITS tables into guest RAM when 'shutdown' the
Or do you think this error is normal?
yes we determined in the past this was the right moment do save the

"with a live migration the guest is still running after
the RAM has been first saved, and so the tables may still change
during the iterative RAM save. You would actually need to do this
at just the point we stop the VM before the final RAM save; that
be possible by using a notifier hook a vm run state change to
- if you can do the changes just as the migration flips into that mode
it *should* work. " said David.

But sometimes as the commit msg says, the VM is stopped before the
migration launch - I do not remember the exact scenario tbh -.
Well, I initially wanted to know more about this scenario to determine
a normal shutdown would fall into it.😂
I think it was for save/restore use case. In that case you need to flush
the KVM cache in memory on VM shutdown.
Sorry for late reply.

Can we distinguish from the 'RunState'?
When we stop the VM, the RunState will be set. There are many types of

Maybe RUN_STATE_SHUTDOWN doesn't belong to save/restore use case,
Adding Dave, Juan and Peter in the loop for migration expertise.

At the moment we save the ARM ITS MSI controller tables whenever the VM
gets stopped. Saving the tables from KVM caches into the guest RAM is
needed for migration and save/restore use cases.
However with GICv4 this fails at KVM level because some MSIs are
forwarded and saving their state is not supported with GICv4.

While GICv4 migration is not supported we would like the VM to work
properly, ie. being stoppable without taking care of table saving.

So could we be more precise and identifiy the save/restore and migration
use cases instead of saving the tables on each VM shutdown.
During the precopy migration (not sure about others), we do:

static void migration_completion(MigrationState *s)
             ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
                 ret = qemu_savevm_state_complete_precopy(s->to_dst_file, false,

so I think we do have that state there to hook off.
That's consistent with what you suggested in the past ans what is logged
in the commit message of

cddafd8f353d2d251b1a5c6c948a577a85838582 ("hw/intc/arm_gicv3_its: Implement state 

However does the save/restore enters that state. If I remember correctly that's 
why I decided to do the save on each VM stop instead.

My idea is to distinguish between save/restore, migration use cases and
other scenarios based on the 'RunState'. RUN_STATE_FINISH_MIGRATION and
RUN_STATE_COLO should be the right cases, I think. I'm not sure
RUN_STATE_SAVE_VM is a right case, which is used in save_snapshot.

Do you have a better way to identify it?

By the way, it's just get an -EACCESS error and doesn't cause any other
problems. If we don't have a good and easy way to solve it, maybe we can
ignore it.

Kunkun Jiang

The tables are saved into guest RAM so when need the CPUs and devices to
be stopped but we need the guest RAM to be saved after the ITS save
Yeh so what should happen is that you:
    a) Iterate RAM a lot
    b) You stop everything
      -> Flushes remaining changes into RAM
    c) Transmit device state and last bits of RAM changes.

so that flush should happen at (b).
That's correct.



Kunkun, by the way you currently just get an error from qemu, ie. qemu
does not exit? Couldn't we just ignore -EACCESS error?



In my opinion, when the virtual machine is normally shutdown, flushing
ITS tables is not necessary. If we can't tell the difference between
'normal shutdown'
and the above scenario, then this 'error' is inevitable.
So each time the VM is stopped we flush the caches into guest RAM.

This error occurs in the following scenario:
Kunpeng 920 、enable GICv4、passthrough a accelerator Hisilicon SEC
the VM.

The flow is as follows:

               vm_change_state_handler (hw/intc/arm_gicv3_its_kvm.c)


There is such a code in vgic_its_save_itt():
    * If an LPI carries the HW bit, this means that this
    * interrupt is controlled by GICv4, and we do not
    * have direct access to that state without GICv4.1.
    * Let's simply fail the save operation...
if (ite->irq->hw && !kvm_vgic_global_state.has_gicv4_1)
             return -EACCES;
Maybe we miss a piece of code for 4.0 that unsets the forwarding. The
only way to handle this is to make sure  ite->irq->hw is not set on
shutdown, no?
It's not going to return -EACCES here, if we unset the forwarding
first. But
this may cause problems in save/restore scenarios. The GICv4 architecture
doesn't give any guarantee that the pending state is written into the
pending table.

Kunkun Jiang

As far as I understand you need a v4.1 to migrate,
following Shenming's series
[PATCH v5 0/6] KVM: arm64: Add VLPI migration support on GICv4.1
Maybe sync with him?
Yes, GICv4 does not support live migrate.

Kunkun Jiang


Looking forward to your reply.

Kunkun Jiang



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