Re: GPU passthrough issue when VM is configured with 4G memory

["Pasi Kärkkäinen" <pasik@iki.fi>]

On Fri, May 17, 2013 at 07:10:17AM +0000, Hanweidong wrote:
> 
> > -----Original Message-----
> > From: Pasi Kärkkäinen [mailto:pasik@iki.fi]
> > Sent: 2013???5???10??? 0:50
> > To: Hanweidong
> > Cc: George Dunlap; Stefano Stabellini; xudong.hao@intel.com;
> > Yanqiangjun; Luonengjun; Wangzhenguo; Yangxiaowei; Gonglei (Arei);
> > Anthony Perard; xen-devel@lists.xen.org; xiantao.zhang@intel.com
> > Subject: Re: [Xen-devel] GPU passthrough issue when VM is configured
> > with 4G memory
> > 
> > On Thu, Apr 25, 2013 at 02:24:11PM +0000, Hanweidong wrote:
> > > >
> > > > So just to check to make sure I understand correctly: the problem
> > is
> > > > that qemu has mapped the guests' pages before the memory hole is
> > made
> > > > larger?
> > > >
> > >
> > > Yes.
> > >
> > > > In that case, wouldn't it make sense to flush qemu's mapcache
> > whenever
> > > > the memory layout changes?  Such a thing should be possible now,
> > e.g.,
> > > > when doing ballooning, right?
> > > >
> > >
> > > Good point. Will have a try.
> > >
> > 
> > Any luck with debugging this issue?
> 
> Flushing qemu's mapcache doesn't help this issue. Unlike ballooning, this case needs to adjust memory region layout. Seems it also needs to update ram regions besides pci hole region, don't know it's feasible to update ram regions (initialized in xen_ram_init()) when getting actual pci hole start address from hvmloader.
> 

Ok. 

> > 
> > I'm setting up a new box for testing GPU passthrough so I'm happy to
> > test
> > any patches you might have..
> > 
> 
> Actually, it's not really related to GPU passthrough. You can reproduce it when a VM has a big pci hole size (such as 512MB), e.g. create a VM with a virtual device which has a 512MB pci BAR.
> 

George: Should we track this issue in the 4.3 status emails? It seems it's a generic PCI passthrough issue.


-- Pasi

> Weidong
>  
> > Thanks,
> > 
> > -- Pasi
>