On Mon, 23 Apr 2018 10:05:54 -0300 Eduardo Habkost <ehabk...@redhat.com> wrote:
> On Mon, Apr 23, 2018 at 11:50:16AM +0200, Igor Mammedov wrote: > > On Fri, 20 Apr 2018 08:31:18 +0200 > > Markus Armbruster <arm...@redhat.com> wrote: > > > > > Eduardo Habkost <ehabk...@redhat.com> writes: > > > > > > > On Thu, Apr 19, 2018 at 10:00:04AM +0200, Igor Mammedov wrote: > > > >> On Wed, 18 Apr 2018 09:08:30 +0200 > > > >> Markus Armbruster <arm...@redhat.com> wrote: > > > >> > > > >> > Eduardo Habkost <ehabk...@redhat.com> writes: > > > >> > > > > >> > > On Tue, Apr 17, 2018 at 05:41:10PM +0200, Igor Mammedov wrote: > > > >> > > > > > >> > >> On Tue, 17 Apr 2018 11:27:39 -0300 > > > >> > >> Eduardo Habkost <ehabk...@redhat.com> wrote: > > > >> > >> > > > >> > >> > On Tue, Apr 17, 2018 at 04:13:34PM +0200, Markus Armbruster > > > >> > >> > wrote: > > > >> > >> > > Igor Mammedov <imamm...@redhat.com> writes: > > > >> > >> > > > > > >> > >> > > [...] > > > >> > >> > > > Series allows to configure NUMA mapping at runtime using QMP > > > >> > >> > > > interface. For that to happen it introduces a new > > > >> > >> > > > '-preconfig' CLI option > > > >> > >> > > > which allows to pause QEMU before machine_init() is run and > > > >> > >> > > > adds new set-numa-node QMP command which in conjunction with > > > >> > >> > > > query-hotpluggable-cpus allows to configure NUMA mapping > > > >> > >> > > > for cpus. > > > >> > >> > > > > > > >> > >> > > > Later we can modify other commands to run early, for > > > >> > >> > > > example device_add. > > > >> > >> > > > I recall SPAPR had problem when libvirt started QEMU with > > > >> > >> > > > -S and, while it's > > > >> > >> > > > paused, added CPUs with device_add. Intent was to coldplug > > > >> > >> > > > CPUs (but at that > > > >> > >> > > > stage it's considered hotplug already), so SPAPR had to > > > >> > >> > > > work around the issue. > > > >> > >> > > > > > >> > >> > > That instance is just stupidity / laziness, I think: we > > > >> > >> > > consider any > > > >> > >> > > plug after machine creation a hot plug. Real machines remain > > > >> > >> > > cold until > > > >> > >> > > you press the power button. Our virtual machines should > > > >> > >> > > remain cold > > > >> > >> > > until they start running, i.e. with -S until the first > > > >> > >> > > "cont". > > > >> > >> It probably would be too risky to change semantics of -S from > > > >> > >> hotplug to coldplug. > > > >> > >> But even if we were easy it won't matter in case if dynamic > > > >> > >> configuration > > > >> > >> done properly. More on it below. > > > >> > >> > > > >> > >> > > I vaguely remember me asking this before, but your answer > > > >> > >> > > didn't make it > > > >> > >> > > into this cover letter, which gives me a pretext to ask again > > > >> > >> > > instead of > > > >> > >> > > looking it up in the archives: what exactly prevents us from > > > >> > >> > > keeping the > > > >> > >> > > machine cold enough for numa configuration until the first > > > >> > >> > > "cont"? > > > >> > >> > > > > >> > >> > I also think this would be better, but it seems to be difficult > > > >> > >> > in practice, see: > > > >> > >> > http://mid.mail-archive.com/20180323210532.GD28161@localhost.localdomain > > > >> > >> > > > > >> > >> > > > >> > >> In addition to Eduardo's reply, here is what I've answered back > > > >> > >> when you've asked question the 1st time (v2 late at -S pause > > > >> > >> point reconfig): > > > >> > >> https://www.mail-archive.com/qemu-devel@nongnu.org/msg504140.html > > > >> > >> > > > >> > >> In short: > > > >> > >> I think it's wrong in general doing fixups after machine is build > > > >> > >> instead of getting correct configuration before building machine. > > > >> > >> That's going to be complex and fragile and might be hard to do at > > > >> > >> all depending on what we are fixing up. > > > >> > > > > > >> > > What "building the machine" should mean, exactly, for external > > > >> > > users? > > > >> under "building machine", I've meant machine_run_board_init() > > > >> and all follow up steps to machine_done stage. > > > >> > > > >> > > The main question I'd like to see answered is: why exactly we > > > >> > > must "build" the machine before the first "cont" is issued when > > > >> > > using -S? Why can't we delay everything to "cont" when using -S? > > > >> > > > > > >> Nor sure what question is about, > > > >> Did you mean if it were possible to postpone machine_run_board_init() > > > >> and all later steps to -S/cont time? > > (1) > > As David said -S pause point is practically breakpoint on some > > instruction of built/existing machine and current monitor commands > > expect it to be valid. Moving -S before machine_run_board_init() > > will break semantics of current -S pause point (i.e. user expectation > > on existing machine) as well as most of the commands that evolved > > in environment where machine already existed. > > OK, so what's missing here is a clear description what the user > can expect on -S. Currently it's fully configured machine with all CLI options taken in account in paused state in initial state or with state it is getting from migration stream if -incoming were used in combination with -S. > > Hence a new -preconfig option and runstate to avoid breaking > > exiting users and being able to cleanly handle configuration that > > affects machine_run_board_init(). > > > > > > Exactly. In other words, what exactly must be done before the > > > > monitor is available when using -S, > > for MUST, it should be commands that affect machine_run_board_init() > > like being added set-numa-node > > > > > > and what exactly can be postponed after "cont" when using -S? > > hotplug configuration and various runtime query commands that > > expect built machine. (today it's most of the commands) > > > > wrt configuration commands we should split them into coldplug > > and hotplug ones (some could be both). > > > > > >> > > Is it just because it's a long and complex task? Does that mean > > > >> > > we might still do that eventually, and eliminate the > > > >> > > prelaunch/preconfig distinction in the distant future? > > > >> > > > > >> > Why would anyone want to use -S going forward? For reasons other > > > >> > "we've > > > >> > always used -S, and can't be bothered to change". > > > >> We should be able to deprecate/remove -S once we can do all > > > >> initial configuration that's possible to do there at > > > >> preconfig time. > > > > > > This sounds like there are things we can do with -S but can't > > > --preconfig now. Is that correct? > > yes, we can't do at --preconfig time anything that requires built machine. > > "built machine" is a very broad description. We need to specify > more clearly what "built machine" means for an external user. > Does it mean having the QOM tree available? Does it mean having > the VCPU threads created? Without defining what -S really must > provide, we won't be able to deprecate and replace it. (*2) how about s/built machine/machine ready to execute guest code/, that's what it is now. > > > > If the plan is to deprecate -S, what are the important > > > > user-visible differences between -S and -preconfig today? Do we > > > > plan to eliminate all those differences before > > > > deprecating/removing -S? > > we probably won't be able to deprecate -S in foreseeable future, > > for that we would need to be able to do everything starting from > > machine_run_board_init() to current pause point. > > But we can gradually move configuration commands to -preconfig time > > and gradually add CLI equivalents for that aren't possible at -S time > > (like Paolo suggested picking to be used machine model at runtime) > > This could be a good plan, if we can explain why exactly -S is > still needed. For a while -S would be need at least for compat reasons, if we ever get to point where at -preconfig time machine could be build up to the point -S provides[2] then we can talk about deprecating it, for now it's way too premature to do something about it /I mean documenting intent which is not there yet and might never materialize as there is no real demand to deprecate it/. > [...] > > > >> But I've been sitting on these patches for > > > >> a long time and what's obvious to me might be not so clear to others. > > > >> > > > > > > Par for the course, don't feel bad about it. > > > > > > >> I might just not see what's missing. Any suggestions to improve it > > > >> are welcome. > > > > > > > > I miss something that documents why both -S and -preconfig need > > > > to exist, what are the differences between them today, and what > > > > we plan to do about the differences between them in the future. > > Where would you prefer it being documented? > > I suggest qemu-options.hx and/or qemu-doc.texi. Regarding qemu-options.hx patch "[PATCH for-2.13 v5 03/11] cli: add --preconfig option" adds doc text describing --preconfig option with explanation of how 'cont' could be used (including in combination with -S). I'll try to come up with a text for qemu-doc.texi, not about deprecating -S but about when --preconfig should be used vs -S and where to get list of commands that could be used at preconfig state. > BTW, "cont" is documented as "Resume guest VCPU execution", which > is not true when using preconfig. Maybe it's better to add a > separate QMP command for "create machine and devices" instead of > overloading the semantics of "cont"? My bad, I've missed it, I can fixup 'cont' description to match its behavior with --preconfig taken in account. I'm not so sure about adding a new command is better though, I recall Markus being against adding new commands unless we have to, but I don't have strong inclination both ways so it's up to you. I'm more inclined towards reusing 'cont', it seems logical (/me looking from the point if I were user).
