Re: [libvirt] [PATCH v3 3/4] qemu_capabilities: Introduce QEMU_CAPS_PM_WAKEUP_SUPPORT

On 4/11/19 10:39 AM, Peter Krempa wrote: > On Thu, Apr 11, 2019 at 10:25:11 +0200, Michal Privoznik wrote: >> This capability tells whether qemu is capable of waking up the >> guest from PM suspend. >> >> Based-on-work-of: Daniel Henrique Barboza <danielhb413(a)gmail.com&gt; >> Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; >> --- >>   src/qemu/qemu_capabilities.c                  | 24 >> +++++++++++++++++++ >>   src/qemu/qemu_capabilities.h                  |  3 +++ >>   .../caps_4.0.0.riscv32.replies                | 11 +++++++++ >>   .../caps_4.0.0.riscv32.xml                    |  1 + >>   .../caps_4.0.0.riscv64.replies                | 11 +++++++++ >>   .../caps_4.0.0.riscv64.xml                    |  1 + >>   .../caps_4.0.0.x86_64.replies                 | 11 +++++++++ >>   .../caps_4.0.0.x86_64.xml                     |  1 + >>   8 files changed, 63 insertions(+) >> > > [...] > > >>   bool >>   virQEMUCapsCPUFilterFeatures(const char *name, >>                                void *opaque) >> @@ -4373,6 +4395,8 @@ virQEMUCapsInitQMPMonitor(virQEMUCapsPtr >> qemuCaps, >>           return -1; >>       if (virQEMUCapsProbeQMPSEVCapabilities(qemuCaps, mon) < 0) >>           return -1; >> +    if (virQEMUCapsProbeQMPCurrentMachine(qemuCaps, mon) < 0) >> +        return -1; > > This seems wrong by definition. The function is supposed to query > current machine, but the capability lookup code uses 'none' machine > type. IIUC the support for wakeup in some cases depends on the presence > of ACPI in the guest and thus really can't be cached this way. Yep, I was just about to write this but then got sidetracked. So I see two options here: 1) Query if guest is capable of PM suspend at runtime. However, this has to be done in a clever way. It would need to be done not only in qemuProcessLaunch() (or some subordinate functions), it would need to be done in qemuProcessReconnect() too (to cover case when user is upgrading libvirt). For this, qemuMonitorConnect() looks like the best place - it's called from both locations and it also already contains some 'runtime capabilities' querying. Except not really - the migration capabilities queried there are stored in @priv->migrationCaps rather than priv->qemuCaps. Problem with storing runtime capabilities in priv->qemuCaps is that they would get formatted into status XML. And this opens whole different can of worms. For instance, this feature relies on 'query-commands' to see if 'query-current-machine' command is available. Well, we can't re-run 'query-commands' in runtime because that might change the set of qemuCaps used when constructing cmd line. Way around this is to have 'runtime only' capabilities, that would not be stored in status XML. At some point we will need those, but writing that infrastructure is not easy and a bit too much to ask for this small fix. Especially, when we have another option: 2) Issue 'query-current-machine' from qemuDomainPMSuspendForDuration(). In every run. This is a bit suboptimal, but how frequently do we expect this API to be called? Caching qemu capabilities is there to speed up decission process which makes sense if we'd be making the decission 1000 times a second (e.g. building cmd line). With this approach the slowdown would be negligible. Of course, we would need to run the command in 'silent' mode, i.e. not report any error if qemu doesn't know it.

Michal