From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 23 Jan 2023 18:27:23 +0000 From: "Dr. David Alan Gilbert" Message-ID: References: <20230115170903.3416105-1-antonkuchin@yandex-team.ru> <20230119074602-mutt-send-email-mst@kernel.org> <20230120085534-mutt-send-email-mst@kernel.org> <703d527f-de92-090c-6ce1-af0dec7de033@yandex-team.ru> <20230122030455-mutt-send-email-mst@kernel.org> <20230122093903-mutt-send-email-mst@kernel.org> <70c0f00a-7828-3ccf-c2ea-49aeef8693e9@yandex-team.ru> <20230122111618-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: <20230122111618-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset="iso-8859-1" Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Subject: Re: [Virtio-fs] [PATCH] vhost-user-fs: add capability to allow migration List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: Anton Kuchin , qemu-devel@nongnu.org, virtio-fs@redhat.com, Markus Armbruster , Eric Blake , Stefan Hajnoczi , Juan Quintela , yc-core@yandex-team.ru * Michael S. Tsirkin (mst@redhat.com) wrote: > On Sun, Jan 22, 2023 at 06:09:40PM +0200, Anton Kuchin wrote: > >=20 > > On 22/01/2023 16:46, Michael S. Tsirkin wrote: > > > On Sun, Jan 22, 2023 at 02:36:04PM +0200, Anton Kuchin wrote: > > > > > > This flag should be set when qemu don't need to worry about any > > > > > > external state stored in vhost-user daemons during migration: > > > > > > don't fail migration, just pack generic virtio device states to > > > > > > migration stream and orchestrator guarantees that the rest of t= he > > > > > > state will be present at the destination to restore full contex= t and > > > > > > continue running. > > > > > Sorry I still do not get it. So fundamentally, why do we need t= his property? > > > > > vhost-user-fs is not created by default that we'd then need opt-i= n to > > > > > the special "migrateable" case. > > > > > That's why I said it might make some sense as a device property a= s qemu > > > > > tracks whether device is unplugged for us. > > > > >=20 > > > > > But as written, if you are going to teach the orchestrator about > > > > > vhost-user-fs and its special needs, just teach it when to migrat= e and > > > > > where not to migrate. > > > > >=20 > > > > > Either we describe the special situation to qemu and let qemu > > > > > make an intelligent decision whether to allow migration, > > > > > or we trust the orchestrator. And if it's the latter, then 'migra= te' > > > > > already says orchestrator decided to migrate. > > > > The problem I'm trying to solve is that most of vhost-user devices > > > > now block migration in qemu. And this makes sense since qemu can't > > > > extract and transfer backend daemon state. But this prevents us from > > > > updating qemu executable via local migration. So this flag is > > > > intended more as a safety check that says "I know what I'm doing". > > > >=20 > > > > I agree that it is not really necessary if we trust the orchestrator > > > > to request migration only when the migration can be performed in a > > > > safe way. But changing the current behavior of vhost-user-fs from > > > > "always blocks migration" to "migrates partial state whenever > > > > orchestrator requests it" seems a little=A0 dangerous and can be > > > > misinterpreted as full support for migration in all cases. > > > It's not really different from block is it? orchestrator has to arran= ge > > > for backend migration. I think we just assumed there's no use-case wh= ere > > > this is practical for vhost-user-fs so we blocked it. > > > But in any case it's orchestrator's responsibility. > >=20 > > Yes, you are right. So do you think we should just drop the blocker > > without adding a new flag? >=20 > I'd be inclined to. I am curious what do dgilbert and stefanha think thou= gh. Yes I think that's probably OK, as long as we use the flag for knowing how to handle the discard bitmap as a proxy for the daemon knowing how to handle *some* migrations; knowing which migrations is then the job for the orchestrator to be careful of. Dave > --=20 > MST >=20 --=20 Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK