Re: [PULL 07/42] machine: aux-ram-share option

[Peter Maydell <peter.maydell@linaro.org>]

On Wed, 5 Nov 2025 at 19:13, Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Nov 04, 2025 at 10:53:54AM +0000, Peter Maydell wrote:
> > On Wed, 29 Jan 2025 at 16:03, Fabiano Rosas <farosas@suse.de> wrote:
> > >
> > > From: Steve Sistare <steven.sistare@oracle.com>
> > >
> > > Allocate auxilliary guest RAM as an anonymous file that is shareable
> > > with an external process.  This option applies to memory allocated as
> > > a side effect of creating various devices. It does not apply to
> > > memory-backend-objects, whether explicitly specified on the command
> > > line, or implicitly created by the -m command line option.
> > >
> > > This option is intended to support new migration modes, in which the
> > > memory region can be transferred in place to a new QEMU process, by sending
> > > the memfd file descriptor to the process.  Memory contents are preserved,
> > > and if the mode also transfers device descriptors, then pages that are
> > > locked in memory for DMA remain locked.  This behavior is a pre-requisite
> > > for supporting vfio, vdpa, and iommufd devices with the new modes.
> >
> > Hi; I've just noticed that in this patch:
> >
> >
> > > @@ -1162,6 +1178,12 @@ static void machine_class_init(ObjectClass *oc, void *data)
> > >      object_class_property_set_description(oc, "mem-merge",
> > >          "Enable/disable memory merge support");
> > >
> > > +#ifdef CONFIG_POSIX
> > > +    object_class_property_add_bool(oc, "aux-ram-share",
> > > +                                   machine_get_aux_ram_share,
> > > +                                   machine_set_aux_ram_share);
> > > +#endif
> >
> > we added a new class property to the machine, but we don't
> > call object_class_property_set_description() to give it any
> > help text (compare how we handle the other properties in this
> > function).
> >
> > > +
> > >      object_class_property_add_bool(oc, "usb",
> > >          machine_get_usb, machine_set_usb);
> > >      object_class_property_set_description(oc, "usb",
> >
> > This means that if you run "qemu-system-x86_64 -M q35,help"
> > you'll see that this option is missing help text:
> >
> > pc-q35-10.2-machine options:
> >   acpi=<OnOffAuto>       - Enable ACPI
> >   append=<string>        - Linux kernel command line
> >   aux-ram-share=<bool>
> >   boot=<BootConfiguration> - Boot configuration
> >   bus-lock-ratelimit=<uint64_t> - Set the ratelimit for the bus locks
> > acquired in VMs
> >   confidential-guest-support=<link<confidential-guest-support>> - Set
> > confidential guest scheme to support
> >   default-bus-bypass-iommu=<bool>
> > [etc]
> >
> > Would somebody like to write a patch to add the missing
> > description ?
>
> Thanks for reporting, sent "[PATCH] machine: Provide a description for
> aux-ram-share property" just now.
>
> PS: for q35 there's another one fd-bootchk=..

Yes; it was looking at the bug report about that
https://gitlab.com/qemu-project/qemu/-/issues/3193
that led me to spot the aux-ram-share option.

-- PMM