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From: | Michael Roth |
Subject: | Re: [Qemu-devel] [PATCH v4 0/2]: qemu-ga: Add the guest-suspend command |
Date: | Thu, 05 Jan 2012 15:41:33 -0600 |
User-agent: | Mozilla/5.0 (X11; Linux i686 on x86_64; rv:8.0) Gecko/20111105 Thunderbird/8.0 |
On 01/05/2012 02:25 PM, Luiz Capitulino wrote:
On Thu, 05 Jan 2012 09:10:50 -0600 Michael Roth<address@hidden> wrote:On 01/05/2012 08:42 AM, Luiz Capitulino wrote:On Thu, 5 Jan 2012 12:59:27 +0000 "Daniel P. Berrange"<address@hidden> wrote:On Thu, Jan 05, 2012 at 10:37:14AM -0200, Luiz Capitulino wrote:On Thu, 5 Jan 2012 10:16:30 +0000 "Daniel P. Berrange"<address@hidden> wrote:On Wed, Jan 04, 2012 at 05:45:11PM -0200, Luiz Capitulino wrote:This version drops modes 'sleep' and 'hybrid' because they don't work properly due to issues in qemu. Only the 'hibernate' mode is supported for now.IMHO this is short-sighted. When the bugs QEMU in are fixed so that these modes work, you have needlessly put users in the situation where they have to now upgrade the guest agent everywhere to take advantage of the bugfix.That was my thinking until v4. But after discussing with Michael the issues we have with S3 I concluded that it doesn't make sense to offer an API to something that doesn't work, this will just generate bug reports. Also, updating to get new features is normal and expected.This is assuming that users will always upgrade their VMs& hosts in lock step, which I rather doubt they will in practice. eg imagine a deployment might have a mixture of hosts, running QEMU 1.1 (broken S3) and QEMU 1.2 (working S3). If they build VM disk images they will likely use the QEMU GA from 1.2 for all their builds, even if many of them will only run on QEMU 1.1 hosts. So you'll end up having 'sleep' and 'hybrid' commands available in the guest agent, even though the host QEMU doesn't work properly. So you *will* ultimately need to make sure that QEMU GA from 1.2, has sensible behaviour when run on a QEMU 1.1 host. If you don't address this during 1.1, you may well find yourself in an un-winnable situation for 1.2, where it is impossible to provide good behaviour on old hosts. So IMHO we are better off in the long run, if we include all commands right now, even though some don't work yet, and work to ensure we have good error reporting behaviour for those that don't work.Yes, I agree. As a side note: if we add error reporting it will only work on 1.1 and later. Ie, the problem you describe above will still happen with 1.0. But what you're suggesting seems to be the right thing to do. Do you agree Michael?Agree, but unless we add an RPC that QEMU uses to advertise capabilities, I'm really not sure it's possible to detect whether or not the host will support it.You mean an RPC to advertise if 'sleep' is supported? I think this is best done by making guest-suspend return an error as suggested by Daniel, otherwise a client that doesn't query for capabilities might run in trouble.
Agreed, but what I mean is that if the user executes the suspend using on up-level agent running on a down-level 1.0 host, the agent will still see s3 advertised and issue the buggy suspend. That's why I suggested the host->agent capabilities reporting as a possible (but somewhat ugly) way to just simply tell the agent it can handle it (and, lacking that, assume that it can't).
There's an important detail though: we need to make qemu not advertise S3 for this to work. However, we might be able to fix S3 for 1.1 (and bugs, like the S4 ones, can't be detected, limiting the scope of the 'unsupported' error). So, we could merge all modes and commit to get S3 fixed for 1.1 :)
No disagreement there, if we can commit to making qemu-ga/qemu 1.1 releases interoperable in this manner (whether by fixing s3 or not advertising it), I think that approach is perfectly fine, ideal even. Doing a 1.1 release where qemu and qemu-ga are not interoperable (qemu missing s3 support, qemu-ga using s3) was my main objection.
But there is a 2nd topic here I'm trying to mull over: what is qemu-ga's support policy for down-level hosts? backward-compatible? incompatible?
The above approach to this problem suggests the latter (qemu-ga 1.1 has RPCs that will knowingly break 1.0 qemu instances). We could solve this by introducing the capabilities negotiation I mentioned early. It actually wouldn't need to be anything other than qemu telling qemu-ga what qemu-ga version-level it supports. By default we assume 1.0, and limit qemu-ga to that until qemu-ga is told otherwise (so, no sleep/hybrid suspend modes). For new RPCs we may be able to handle this version automatically, since we include qemu version levels for the RPCs in the schema. For functionality within an RPC (like sleep/hybrid suspend modes) we could use conditional code.
If we take that approach (maintaining backward-compatibility), we'd need to introduce that code in the agent now, and require qemu/libvirt execute the guest-set-support-level RPC or whatever to access these 1.1 features.
Technically, there's a required RPC qemu-ga clients need to execute already: guest-sync. It's required because we have no way to reliably detect EOF over virtio-serial, and thus an agent may send stale data to a newly-connected qemu-ga client, so the client needs to do the guest-sync command to find the expected response and re-sync the streams. We could roll the guest-set-support-level functionality into that. Basically just add another field.
And if we can't detect that reliably, we're better off leaving it out for now, because sleeping guests is not obscure functionality, and accidentally nuking guests when a user sleeps them (presumably because they want to retain their working state) is much worse than telling a user to upgrade their agent, or not supported or whatever.As an example, if S3 is broken in current QEMU, then we should not be advertizing S3 to the guest OS. This would allow 'pm-is-supported --suspend' to return false, at which point the guest agent can send back a nice error message 'Suspend is not supported on this host', instead of just having the guest try to suspend& hang or worse.
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