Re: [SeaBIOS] [Qemu-devel] [RFC PATCH v4 00/30] ACPI memory hotplug

[liu ping fan]

On Thu, Mar 28, 2013 at 5:29 PM, Vasilis Liaskovitis
<[e-mail]> wrote:
> Hi,
>
> [...]
> > > > > I haven't updated the series for a while, but I can rework if there is a more
> > > > > clear direction for the community.
> > > > >
> > > > > Another open issue is reference counting of memoryregions in qemu memory
> > > > > model. In order to make memory hot-remove operations safe, we need to remove
> > > > > a memoryregion after all users (e.g. both guest and block layer) have stopped
> > > > > using it,
> > > >
> > > > it seems it mostly up to the user who want to hot-(un)plug,
> > > > if user want to un-plug a memory which is kernel's main memory, kernel
> > > > will always run on it(never stop) unless power off.
> > > > and if guest stops, all DIMMs should be safe to hot-remove,
> > > > or else we should do something to let user can unlock all reference.
> > >
> > > it's not only the guest-side that needs to stop using it, we need to make sure
> > > that the qemu block layer is also not using the memory region anymore. See the 2
> > > links below for discussion:
> >
> > can't we simply track this(MemoryRegion) usage by ref-count?
> > e.g.
> > every time mr used, inc ref-count, then dec it when unused
> > even for cpu_physical_memory_map and other potential users.
>
> yes, that's the idea the patchset below try to implement, but last
> time I checked this was not upstreamed. I will take a closer look next
> week.

The current  problem is that if not all the device convert to support
MemoryRegionOps ref/unref API, the code in address_space_rw will be
ugly.
But I think the virtio data plane will need thread-safe RAM, so can we
just try that for RAM device.
If it is needed, I will update my patches

Regards,
Pingfan
> > > > > see discussion at
> > > > > http://lists.gnu.org/archive/html/qemu-devel/2012-10/msg03986.html. There was a
> > > > > relevant ibm patchset
> > > > > https://lists.gnu.org/archive/html/qemu-devel/2012-11/msg02697.html
> > > > > but it was not merged.
>
> - Vasilis