On Tue, Nov 22, 2016 at 01:26:49PM +1100, Alexey Kardashevskiy wrote: > On 22/11/16 00:08, Andrea Bolognani wrote: > > On Mon, 2016-11-21 at 13:12 +1100, Alexey Kardashevskiy wrote: > >>>>> 1) switch to PCI Express on newer machine types, and > >>>>> expose some sort of capability through QMP so that > >>>>> libvirt can know about the switch > >>>>> > >>>>> [...] > >>>>> Option 1) would break horribly with existing libvirt > >>>>> versions, and so would Option 2) if we default to using > >>>> > >>>> How exactly 1) will break libvirt? Migrating from pseries-2.7 to > >>>> pseries-2.8 does not work anyway, and machines are allowed to behave > >>>> different from version to version, what distinct difference will using > >>>> "pseries-pcie-X.Y" make? > >>> > >>> Existing libvirt versions assume that pseries guests have > >> > >> If libvirt is using just "pseries" (without a version), then having a > >> virtual PCIe-PCI bridge (and "pci.0" always available by default) will do > >> it. > > > > Please don't. Any device that is included in the guest > > by default and can't be turned off makes libvirt's life > > significantly more difficult (see below). > > > >> If libvirt is using a specific version of pseries, then it already knows > >> that <=2.7 has pci.0 as a root, pcie.0 otherwise. libvirt has a knowledge > >> what QEMU version has what, right? > > > > It doesn't yet, that's the point :) > > > > We *could* add such knowledge to libvirt[1], but *existing* > > libvirt versions would still not know about it, which means > > that upgrading QEMU withough upgrading libvirt will result > > in failure to create new guests. > > > > > >> In what scenario will an additional machine type help? > > > > Because then libvirt could learn that > > > > pseries-x.y <-> pci.0 > > pseries-pcie-x.y <-> pcie.0 > > > > the same way it already knows that > > > > pc-i440fx-x.y <-> pci.0 > > pc-q35-x.y <-> pcie.0 > > > > and choosing between one or the other would be, I think, > > much easier for the user as well. > > > >>> a legacy PCI root bus, and will base their PCI address > >>> allocation / PCI topology decisions on that fact: they > >>> will, for example, use legacy PCI bridges. > >>> > >>> So if you used a new QEMU binary with a libvirt version > >>> that doesn't know about the change, new guests would end up > >>> using the wrong controllers. Existing guests would not be > >>> affected as they would stick with the older machine types, > >>> of course. > >>> > >>>> I believe after we introduced the very first > >>>> pseries-pcie-X.Y, we will just stop adding new pseries-X.Y. > >>> > >>> Isn't i440fx still being updated despite the fact that q35 > >>> exists? Granted, there are a lot more differences between > >>> those two machine types than just the root bus type. > >> > >> I do not know about i440<->q35 but in pseries the difference is going to be > >> very simple. > >> > >> For example, we did not change the machine type when we switched from > >> default OHCI to XHCI, switching from PCI to PCIe does not sound like we > >> need a whole new machine type for this either. > > > > The change from OHCI to XHCI only affected the *default* USB > > controller, which libvirt tries its best not to use anyway: > > instead, it will prefer to use '-M ...,usb=off' along with > > '-device ...' and set both the controller model and its PCI > > address explicitly, partially to shield its users from such > > changes in QEMU. > > > Ok. Always likes this approach really. We should start moving to this > direction with PHB - stop adding the default PHB at all when -nodefaults is > passed (or -machine pseries,pci=off ?) and let libvirt manage PHBs itself > (and provide another spapr-phb type like spapr-pcie-host-bridge or add a > "pcie_root_bus_type" property to the existing PHB type). > > What will be wrong with this approach?
Hm, that's a good point. If were removing the default PHB entirely,
that I would consider a possible case for a new machine type. Putting
construction of the PHBs into libvirt's hand could make life simpler
there. Although it would make it a bit of a pain for people starting
qemu by hand.
I think this option is worth some thought.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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