[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [PATCH] vhost-user-fs: add capability to allow migration
From: |
Juan Quintela |
Subject: |
Re: [PATCH] vhost-user-fs: add capability to allow migration |
Date: |
Wed, 01 Feb 2023 15:26:45 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) |
Anton Kuchin <antonkuchin@yandex-team.ru> wrote:
> On 19/01/2023 18:02, Stefan Hajnoczi wrote:
>> On Thu, 19 Jan 2023 at 10:29, Anton Kuchin <antonkuchin@yandex-team.ru>
>> wrote:
>>> On 19/01/2023 16:30, Stefan Hajnoczi wrote:
>>>> On Thu, 19 Jan 2023 at 07:43, Anton Kuchin <antonkuchin@yandex-team.ru>
>>>> wrote:
>>>>> On 18/01/2023 17:52, Stefan Hajnoczi wrote:
>>>>>> On Sun, 15 Jan 2023 at 12:21, Anton Kuchin <antonkuchin@yandex-team.ru>
>>>>>> wrote:
Hi
Sorry to come so late into the discussion.
>>>>>>> +static int vhost_user_fs_pre_save(void *opaque)
>>>>>>> +{
>>>>>>> + MigrationState *s = migrate_get_current();
>>>>>>> +
>>>>>>> + if (!s->enabled_capabilities[MIGRATION_CAPABILITY_VHOST_USER_FS]) {
>>>>>>> + error_report("Migration of vhost-user-fs devices requires
>>>>>>> internal FUSE "
>>>>>>> + "state of backend to be preserved. If
>>>>>>> orchestrator can "
>>>>>>> + "guarantee this (e.g. dst connects to the same
>>>>>>> backend "
>>>>>>> + "instance or backend state is migrated) set
>>>>>>> 'vhost-user-fs' "
>>>>>>> + "migration capability to true to enable
>>>>>>> migration.");
>>>>>>> + return -1;
>>>>>>> + }
>>>>>>> +
>>>>>>> + return 0;
>>>>>>> +}
>>>>>>> +
>>>>>>> static const VMStateDescription vuf_vmstate = {
>>>>>>> .name = "vhost-user-fs",
>>>>>>> - .unmigratable = 1,
>>>>>>> + .minimum_version_id = 0,
>>>>>>> + .version_id = 0,
>>>>>>> + .fields = (VMStateField[]) {
>>>>>>> + VMSTATE_VIRTIO_DEVICE,
>>>>>>> + VMSTATE_END_OF_LIST()
>>>>>>> + },
>>>>>>> + .pre_save = vhost_user_fs_pre_save,
>>>>>>> };
I don't object to extend the vmstate this way.
But I object to the migration capability for several reasons:
- This feature has _nothing_ to do with migration, the problem, what it
describes, etc is related to vhost_user_fs.
- The number of migration capabilities is limited
And we add checks to see if they are valid, consistent, etc
see qemu/migration/migration.c:migrate_caps_check()
- As Stefan says, we can have several vhost_user_fs devices, and each
one should know if it can migrate or not.
- We have to options for the orchestator:
* it knows that all the vhost_user_fs devices can be migration
Then it can add a property to each vhost_user_fs device
* it don't know it
Then it is a good idea that we are not migrating this VM.
> I think we'd be better without a new marker because migration code
> has standard generic way of solving such puzzles that I described
> above. So adding new marker would go against existing practice.
> But if you could show me where I missed something I'll be grateful
> and will fix it to avoid potential problems.
> I'd also be happy to know the opinion of Dr. David Alan Gilbert.
If everybody agrees that any vhost_user_fs device is going to have a
virtio device, then I agree with you that the marker is not needed at
this point.
Once told that, I think that you are making your live harder in the
future when you add the other migratable devices.
I am assuming here that your "underlying device" is:
enum VhostUserFSType {
VHOST_USER_NO_MIGRATABLE = 0,
// The one we are doing here
VHOST_USER_EXTERNAL_MIGRATABLE,
// The one you describe for the future
VHOST_USER_INTERNAL_MIGRATABLE,
};
struct VHostUserFS {
/*< private >*/
VirtIODevice parent;
VHostUserFSConf conf;
struct vhost_virtqueue *vhost_vqs;
struct vhost_dev vhost_dev;
VhostUserState vhost_user;
VirtQueue **req_vqs;
VirtQueue *hiprio_vq;
int32_t bootindex;
enum migration_type;
/*< public >*/
};
static int vhost_user_fs_pre_save(void *opaque)
{
VHostUserFS *s = opaque;
if (s->migration_type == VHOST_USER_NO_MIGRATABLE)) {
error_report("Migration of vhost-user-fs devices requires internal FUSE
"
"state of backend to be preserved. If orchestrator can "
"guarantee this (e.g. dst connects to the same backend "
"instance or backend state is migrated) set
'vhost-user-fs' "
"migration capability to true to enable migration.");
return -1;
}
if (s->migration_type == VHOST_USER_EXTERNAL_MIGRATABLE) {
return 0;
}
if (s->migration_type == VHOST_USER_INTERNAL_MIGRATABLE) {
error_report("still not implemented");
return -1;
}
assert("we don't reach here");
}
Your initial vmstateDescription
static const VMStateDescription vuf_vmstate = {
.name = "vhost-user-fs",
.unmigratable = 1,
.minimum_version_id = 0,
.version_id = 0,
.fields = (VMStateField[]) {
VMSTATE_INT8(migration_type, struct VHostUserFS),
VMSTATE_VIRTIO_DEVICE,
VMSTATE_END_OF_LIST()
},
.pre_save = vhost_user_fs_pre_save,
};
And later you change it to something like:
static bool vhost_fs_user_internal_state_needed(void *opaque)
{
VHostUserFS *s = opaque;
return s->migration_type == VMOST_USER_INTERNAL_MIGRATABLE;
}
static const VMStateDescription vmstate_vhost_user_fs_internal_sub = {
.name = "vhost-user-fs/internal",
.version_id = 1,
.minimum_version_id = 1,
.needed = &vhost_fs_user_internal_state_needed,
.fields = (VMStateField[]) {
.... // Whatever
VMSTATE_END_OF_LIST()
}
};
static const VMStateDescription vuf_vmstate = {
.name = "vhost-user-fs",
.minimum_version_id = 0,
.version_id = 0,
.fields = (VMStateField[]) {
VMSTATE_INT8(migration_type, struct VHostUserFS),
VMSTATE_VIRTIO_DEVICE,
VMSTATE_END_OF_LIST()
},
.pre_save = vhost_user_fs_pre_save,
.subsections = (const VMStateDescription*[]) {
&vmstate_vhost_user_fs_internal_sub,
NULL
}
};
And you are done.
I will propose to use a property to set migration_type, but I didn't
want to write the code right now.
I think that this proposal will make Stephan happy, and it is just
adding and extra uint8_t that is helpul to implement everything.
Later, Juan.
PD. One of the few things that Pascal got right and C got completely
wrong were pascal variant registers vs C union's. If you have a
union, if should be "required" that there is a field in the
enclosing struct that specifies what element of the union we have.
This is exactly that case.
- Re: [PATCH] vhost-user-fs: add capability to allow migration,
Juan Quintela <=