On 13.11.2023 10:12, Julien Grall wrote: > > > On 13/11/2023 07:37, Jan Beulich wrote: >> On 10.11.2023 18:38, Julien Grall wrote: >>> Hi Jan, >>> >>> On 10/11/2023 12:44, Jan Beulich wrote: >>>> On 10.11.2023 13:23, Roger Pau Monné wrote: >>>>> On Fri, Nov 10, 2023 at 12:34:32PM +0100, Juergen Gross wrote: >>>>>> Get the own domid via creation of a temporary event channel. There is >>>>>> no "official" way to read the own domid in PV guests, so use the event >>>>>> channel interface to get it: >>>>>> >>>>>> - allocate an unbound event channel specifying DOMID_SELF for the >>>>>> other end >>>>>> >>>>>> - read the event channel status which will contain the own domid in >>>>>> unbound.dom >>>>>> >>>>>> - close the event channel >>>>> >>>>> Should we look into introducing a way to expose the domid, so that in >>>>> the future we might not need to resort to this workarounds to get the >>>>> domid? >>>>> >>>>> Maybe in the PV-specific cpuid leaf? It's a shame we didn't put it in >>>>> a non-HVM specific leaf when it was made available to HVM for pvshim >>>>> reasons. >>>> >>>> Couldn't we retroactively generalize the type-agnostic parts of that >>>> leaf? >>> >>> This would only work for x86. I think we want to have a generic >>> hypercalls so it can be used by all arch. >> >> Hmm, yes, perhaps. Otoh it would seem desirable to me if arch-es also >> provided some extension to an arch-natural way of feature detection >> (which CPUID is on x86), without the need to invoke any hypercalls. > > For Arm, I can't really think of anything other than hvc/smc which are > used for calls to the hypervisor/monitor (so basically hypercalls). > > Please suggest if you have a better idea.
I don't know enough Arm to properly suggest something. Arm64 has various id_* system registers, so I would be wondering whether having a properly virtual one reserved in system register space couldn't do the trick. (Clearly just one may not be sufficient long term, so there might be a scalability issue with such an approach. Unlike x86'es CPUID Arm's MRS, aiui, doesn't take inputs other than the coordinates of the system register to read.) Jan
