Re: [Qemu-devel] Windows balloon driver PFN issue

[Michael S. Tsirkin]

On Thu, Feb 01, 2018 at 10:18:53AM +0800, Peter Xu wrote:
> On Wed, Jan 31, 2018 at 04:03:12PM +0200, Michael S. Tsirkin wrote:
> > On Wed, Jan 31, 2018 at 05:28:35PM +0800, Peter Xu wrote:
> > > Hi, Michael and the list,
> > > 
> > > I observed this on windows 8 enterprise guests, when doing memory 
> > > ballooning:
> > > 
> > > address@hidden:virtio_balloon_to_target balloon target: 0x80000000 
> > > num_pages: 524288
> > > address@hidden:virtio_balloon_get_config num_pages: 524288 actual: 0
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 
> > > 0x174604000
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 0x0
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 
> > > 0x17460a000
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 0x0
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 
> > > 0x17460b000
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 0x0
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 
> > > 0x17460c000
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 0x0
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 
> > > 0x17460d000
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 0x0
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 
> > > 0x17460e000
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 0x0
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 
> > > 0x17460f000
> > > address@hidden:virtio_balloon_handle_output section name: pc.ram gpa: 0x0
> > > ...
> > > 
> > > I think it's very possible that these zero addresses (please let me
> > > know what the first 4K page is used for if anyone knows, since IIUC
> > > that's what we throw away now) are half of the 64bit PFN.  Or say, not
> > > sure whether this means a windows guest driver bug that is using
> > > 64bits for PFN rather than 32bits (and I suppose the protocol is using
> > > 32bit for PFNs).
> > > 
> > > Michael, do you know what to do with this?
> > > 
> > > Thanks,
> > 
> > PFN is GPA>>12.  Do you have more than 1<<44 bytes of memory in this VM 
> > then?
> 
> No.  But isn't it still not good to drop the page at offset zero (and
> drop it NNN times)?

Absolutely - looks like a bug. I just don't know why does this happen.

>  And I'm not sure what will happen if guest has
> 1<<44 bytes; then we'll possibly drop very random addresses since a
> real address will be splitted?
> 
> Thanks,

The balloon won't work, period. There was an interface change to fix
that but implementation issues remained and contributor seems to be busy
with page hints.

> -- 
> Peter Xu