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Re: [PATCH v5 2/2] bhyve: support 'isa' controller for LPC



  Daniel P. Berrangé wrote:

> On Thu, Sep 17, 2020 at 05:48:38PM +0400, Roman Bogorodskiy wrote:
> >   Daniel P. Berrangé wrote:
> > 
> > > On Wed, Sep 16, 2020 at 07:14:39PM +0400, Roman Bogorodskiy wrote:
> > > >   Daniel P. Berrangé wrote:
> > > > 
> > > > > On Sat, Aug 01, 2020 at 10:16:57AM +0400, Roman Bogorodskiy wrote:
> > > > > > diff --git a/src/bhyve/bhyve_device.c b/src/bhyve/bhyve_device.c
> > > > > > index fc52280361..52a055f205 100644
> > > > > > --- a/src/bhyve/bhyve_device.c
> > > > > > +++ b/src/bhyve/bhyve_device.c
> > > > > > @@ -46,10 +46,16 @@ bhyveCollectPCIAddress(virDomainDefPtr def G_GNUC_UNUSED,
> > > > > >          if (addr->slot == 0) {
> > > > > >              return 0;
> > > > > >          } else if (addr->slot == 1) {
> > > > > > -            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
> > > > > > -                           _("PCI bus 0 slot 1 is reserved for the implicit "
> > > > > > -                             "LPC PCI-ISA bridge"));
> > > > > > -            return -1;
> > > > > > +            if (!(device->type == VIR_DOMAIN_DEVICE_CONTROLLER &&
> > > > > > +                  device->data.controller->type == VIR_DOMAIN_CONTROLLER_TYPE_ISA)) {
> > > > > > +                 virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
> > > > > > +                                _("PCI bus 0 slot 1 is reserved for the implicit "
> > > > > > +                                  "LPC PCI-ISA bridge"));
> > > > > > +                 return -1;
> > > > > > +            } else {
> > > > > > +                /* We reserve slot 1 for LPC in bhyveAssignDevicePCISlots(), so exit early */
> > > > > > +                return 0;
> > > > > > +            }
> > > > > 
> > > > > I forgot to respond to your followup comments on v4
> > > > > https://www.redhat.com/archives/libvir-list/2020-July/msg01761.html
> > > > > 
> > > > > > > 
> > > > > > > IIUC, this series makes it possible to put the TPC in a different
> > > > > > > slot, so does it still make sense to forbid use of slot 1 as a
> > > > > > > hardcoded rule ?
> > > > > >
> > > > > > IIRC, the idea behind that is to give some time window for users to
> > > > > > allow moving guests from the new version to the old one. If we allow to
> > > > > > use slot 1, it won't be possible to move the guest to the old libvirt as
> > > > > > it will complain slot 1 should be used only for LPC.
> > > > > 
> > > > > If the user has decided to move their LPC to a slot != 1, then it is
> > > > > already impossible to migrate the guest to an old libvirt.
> > > > > 
> > > > > If the user wants to explicitly specify another device in slot 1, then
> > > > > we should not prevent that.
> > > > > 
> > > > > We just need to make sure that if no LPC is in the XML, and no other
> > > > > device explicitly has slot 1, then make sure to auto-assign LPC in slot 1
> > > > > not some other device.
> > > > 
> > > > I've started playing with that and remembered some more corner cases.
> > > > 
> > > > To elaborate on your example, i.e. "no explicit LPC in XML AND no other
> > > > device explicitly on slot 1": these conditions are not specific enough to
> > > > tell whether an LPC device will be added or not.
> > > > 
> > > > In case if the LPC device was not explicitly specified by a user,
> > > > the bhyve driver tries to add it if it's needed (it's required
> > > > for serial console, bootloader, and video devices;
> > > > see bhyveDomainDefNeedsISAController()). Otherwise a domain will start
> > > > without the LPC device.
> > > > 
> > > > This could lead to a case when a user starts a domain in configuration
> > > > that does not require LPC device, but has e.g. a network device on
> > > > a PCI controller that's auto-assigned to slot 1.
> > > > 
> > > > Later user decides to change the configuration and adds a video device,
> > > > which requires LPC. This will lead to addresses changes, as LPC will go
> > > > to slot 1 and a network device's controller will go from slot 1 to slot
> > > > 2, which could be troublesome in some guest OSes.
> > > 
> > > First of all, lets me clear that we're talking about persistent guests
> > > here, not transient guests.
> > > 
> > > With a persistent guest, the PCI addresses are assigned at the time
> > > the XML arrives in virDomainDefineXML. If nothing requires the LPC
> > > at this time, then a NIC could get given slot 1. This is recorded in
> > > the persistent XML.
> > > 
> > > If the user later uses 'virsh edit' to modify the XML and add a video
> > > device, libvirt will see that the NIC is already on slot 1. It will
> > > thus have to give the LPC slot 2 (or whatever is free). The NIC will
> > > not move from slot 1, as that slot is considered taken at this time.
> > > 
> > > The same is true if using virDomainAttachDevice to add a video
> > > card. Any modifications to the XML must never change addresses that
> > > are currently recorded in the XML, only ever place devices in new
> > > unused slots.
> > 
> > Sorry, I should have stated that I was assuming that LPC always
> > gets slot 1 assigned if it has no address explicitly assigned by the user
> > in the XML.
> > 
> > In my understanding, some guests are picky about what slot LPC
> > is assigned to (and it seems that slot 1 and slot 31 are the most common
> > safe options). In this case we're letting user to resolve it in a way
> > they think fits better their specific needs, correct?
> > 
> > In other words, in context of address allocation, we treat LPC like any
> > other device, with the only difference that we start address allocation
> > from it, so it gets slot 1 if it has no address specified and the slot 1
> > is still free?
> 
> Ok, so it sounds like if the user has explicitly used slot 1 for some
> arbitrary device we should allow that, but if libvirt is assigning
> slots, it should never use slot 1 except for the LPC

Going back to my example (slightly adjusted), when the user creates a
configuration that does not require LPC, and explicitly assigns some
other device to slot 1, and then updates the configuration in a way that
LPC becomes required, the LPC device will be assigned to the next free
slot available.

If that doesn't work for this specific configuration, it's fair
to expect user to fix that assuming the user knows what they're doing by
interfering into address allocations. The fix will be a trivial swap of
devices between slot 1 and the slot allocated for LPC, without affecting
other device addresses.

This looks like a logical and expected behavior to me, I'll update the
series accordingly.

Thanks

Roman Bogorodskiy

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