Re: [RFC PATCH 00/19] GICv4 Support for Xen

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Mykyta,

We have a number of series from you which have not been merged yet and
reviewing them all in parallel might be challenging.

Would you mind giving us a status and maybe priorities on them.

I could list the following series:
- GICv4
- CPU Hotplug on arm
- PCI enumeration on arm
- IPMMU for pci on arm
- dom0less for pci passthrough on arm
- SR-IOV for pvh
- SMMU for pci on arm
- MSI injection on arm
- suspend to ram on arm

There might be others feel free to complete the list.

On GICv4...

> On 2 Feb 2026, at 17:14, Mykyta Poturai <Mykyta_Poturai@xxxxxxxx> wrote:
> 
> This series introduces GICv4 direct LPI injection for Xen.
> 
> Direct LPI injection relies on the GIC tracking the mapping between physical 
> and
> virtual CPUs. Each VCPU requires a VPE that is created and registered with the
> GIC via the `VMAPP` ITS command. The GIC is then informed of the current
> VPE-to-PCPU placement by programming `VPENDBASER` and `VPROPBASER` in the
> appropriate redistributor. LPIs are associated with VPEs through the `VMAPTI`
> ITS command, after which the GIC handles delivery without trapping into the
> hypervisor for each interrupt.
> 
> When a VPE is not scheduled but has pending interrupts, the GIC raises a 
> per-VPE
> doorbell LPI. Doorbells are owned by the hypervisor and prompt rescheduling so
> the VPE can drain its pending LPIs.
> 
> Because GICv4 lacks a native doorbell invalidation mechanism, this series
> includes a helper that invalidates doorbell LPIs via synthetic “proxy” 
> devices,
> following the approach used until GICv4.1.
> 
> All of this work is mostly based on the work of Penny Zheng
> <penny.zheng@xxxxxxx> and Luca Fancellu <luca.fancellu@xxxxxxx>. And also from
> Linux patches by Mark Zyngier.
> 
> Some patches are still a little rough and need some styling fixes and more
> testing, as all of them needed to be carved line by line from a giant ~4000 
> line
> patch. This RFC is directed mostly to get a general idea if the proposed
> approach is suitable and OK with everyone. And there is still an open question
> of how to handle Signed-off-by lines for Penny and Luca, since they have not
> indicated their preference yet.

I would like to ask how much performance benefits you could
have with this.
Adding GICv4 support is adding a lot of code which will have to be maintained
and tested and there should be a good improvement to justify this.

Did you do some benchmarks ? what are the results ?

At the time where we started to work on that at Arm, we ended up in the 
conclusion
that the complexity in Xen compared to the benefit was not justifying it hence 
why
this work was stopped in favor of other features that we thought would be more
beneficial to Xen (like PCI passthrough or SMMUv3).

Cheers
Bertrand