On Wed, May 07, 2025 at 08:44:05PM +0000, Matt Ochs wrote: > Hi Daniel, > > Thanks for your feedback! > > > On May 7, 2025, at 11:51 AM, Daniel P. Berrangé <berra...@redhat.com> wrote: > > On Fri, Apr 11, 2025 at 08:40:54AM -0700, Matthew R. Ochs via Devel wrote: > >> Resending: Series has been re-based over latest upstream. > >> > >> This patch series adds support for configuring the PCI high memory MMIO > >> window size for aarch64 virt machine types. This feature has been merged > >> into the QEMU upstream master branch [1] and will be available in QEMU > >> 10.0. > >> It allows users to configure the size of the high memory MMIO window above > >> 4GB, which is particularly useful for systems with large amounts of PCI > >> memory requirements. > >> > >> The feature is exposed through the domain XML as a new PCI feature: > >> <features> > >> <pci> > >> <highmem-mmio-size unit='G'>512</highmem-mmio-size> > >> </pci> > >> </features> > >> > >> When enabled, this configures the size of the PCI high memory MMIO window > >> via QEMU's highmem-mmio-size machine property. The feature is only > >> available for aarch64 virt machine types and requires QEMU support. > > > > This isn't my area of expertize, but could you give any more background > > on why we need to /manually/ set such a property on Arm only ? Is there > > something that prevents us making QEMU "do the right thing" ? > > The highmem-mmio-size property is only available for the arm64 “virt” > machine. It is only needed when a VM configuration will exceed the 512G > default for PCI highmem region. There are some GPU devices that exist > today that have very large BARs and require more than 512G when > multiple devices are passed through to a VM. > > Regarding making QEMU “do the right thing”, we could add logic to > libvirt to detect when these known devices are present in the VM > configuration and automatically set an appropriate size for the > parameter. However I was under the impression that type of solution > was preferred to be handled at the mgmt app layer.
I wasn't suggestnig to put logic in libvirt actually. I'm querying why QEMU's memory map is setup such that this PCI assignment can't work by default with a standard QEMU configuration ? Can you confirm this works correctly on x86 QEMU with q35 machine type by default ? If so, what prevents QEMU 'virt' machine for aarch64 being changed to also work ? Libvirt can't detect when the devices are present in the VM config because this mmio setting is a cold boot option, while PCI devices are often hot-plugged to an existing VM. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
