[libvirt] [PATCH 00/10] conf: Clean up virDomain*Def creation

[Andrea Bolognani]

On Fri, 2017-06-30 at 12:38 +0200, Peter Krempa wrote:
> > Isolation groups are used to make sure any given device ends
> > up on the same bus as related devices and on a different bus
> > as unrelated devices.
> > 
> > They're an abstract concept, and while working on the initial
> > implementation it just happened to be convenient for me to
> > have the isolation group match the IOMMU group. There's no
> > specific reason that has to be the case.
> 
> Fair enough. The documentation you are adding in the linked series is
> vague enough to alow this meaning too:
> 
> @@ -164,6 +164,16 @@ struct _virDomainDeviceInfo {
>       */
>      int pciConnectFlags; /* enum virDomainPCIConnectFlags */
>      char *loadparm;
> +
> +    /* PCI devices will only be automatically placed on a PCI bus
> +     * that shares the same isolation group */
> +    int isolationGroup;
> +
> +    /* Usually, PCI buses will take on the same isolation group
> +     * as the first device that is plugged into them, but in some
> +     * cases we might want to prevent that from happening by
> +     * locking the isolation group */
> +    bool isolationGroupLocked;
>  };

It's vague on purpose :)

All I'm describing there is the interface from the generic
PCI address allocation code's point of view: the fact that
the QEMU driver derives isolation groups from IOMMU groups
is just an implementation detail and as such should not be
mentioned at all.

> > We're never converting back and forth between the two, which
> > I agree would end up in misery at some point down the line;
> > we just set the isolation group once per device and then just
> > perform comparison between isolation groups from there on.
> 
> I'd suggest you create a helper to assign those then (be it from IOMMU
> group or something else), so there's at least a single point that can be
> referenced in the future and which will explain this reasoning.

Good idea, I'll do that!

> Also adding a note that 0 means the device is not isolated would make
> sense in the structure above.

Well, devices *always* get isolated: it's just that all
guests except for pSeries only ever have a single isolation
group ;)

-- 
Andrea Bolognani / Red Hat / Virtualization