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
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