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Re: [PATCH v3 1/1] vhost-user-fs: add migration type property


From: Anton Kuchin
Subject: Re: [PATCH v3 1/1] vhost-user-fs: add migration type property
Date: Wed, 22 Feb 2023 19:05:47 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1

On 22/02/2023 18:51, Michael S. Tsirkin wrote:
On Wed, Feb 22, 2023 at 06:49:10PM +0200, Anton Kuchin wrote:
On 22/02/2023 17:14, Vladimir Sementsov-Ogievskiy wrote:
On 22.02.23 17:25, Anton Kuchin wrote:
+static int vhost_user_fs_pre_save(void *opaque)
+{
+    VHostUserFS *fs = opaque;
+    g_autofree char *path = object_get_canonical_path(OBJECT(fs));
+
+    switch (fs->migration_type) {
+    case VHOST_USER_MIGRATION_TYPE_NONE:
+        error_report("Migration is blocked by device %s", path);
+        break;
+    case VHOST_USER_MIGRATION_TYPE_EXTERNAL:
+        return 0;
+    default:
+        error_report("Migration type '%s' is not
supported by device %s",
+ VhostUserMigrationType_str(fs->migration_type), path);
+        break;
+    }
+
+    return -1;
+}
Should we also add this as .pre_load, to force user select
correct migration_type on target too?
In fact, I would claim we only want pre_load.
When qemu is started on destination we know where it's migrated
from so this flag can be set.
When qemu is started on source we generally do not yet know so
we don't know whether it's safe to set this flag.
But destination is a "source" for next migration, so there shouldn't be
real difference.
The new property has ".realized_set_allowed = true", so, as I understand
it may be changed at any time, so that's not a problem.
Yes, exactly. So destination's property sets not how it will handle this
incoming
migration but the future outgoing one.

How do you know where you are going to migrate though?
I think you don't.
Setting it on source is better since we know where we
are migrating from.

Yes, I don't know where I'm going to migrate to. This is why property
affects only how source saves state on outgoing migration.


This property selects if VM can migrate and if it can what should
qemu put
to the migration stream. So we select on source what type of
migration is
allowed for this VM, destination can't check anything at load time.
OK, so the new field "migration" regulates only outgoing migration and
do nothing for incoming. On incoming migration the migration stream
itself defines the type of device migration.
Worth mentioning in doc?
Good point. I don't think this deserves a respin but if I have to send v4
I'll include
clarification in it.



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