Re: [Xen-devel] GPU passthrough issue when VM is configured with 4G memory

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Fri, May 17, 2013 at 07:10:17AM +0000, Hanweidong wrote:
> 
> > -----Original Message-----
> > From: Pasi Kärkkäinen [mailto:pasik@xxxxxx]
> > Sent: 2013???5???10??? 0:50
> > To: Hanweidong
> > Cc: George Dunlap; Stefano Stabellini; xudong.hao@xxxxxxxxx;
> > Yanqiangjun; Luonengjun; Wangzhenguo; Yangxiaowei; Gonglei (Arei);
> > Anthony Perard; xen-devel@xxxxxxxxxxxxx; xiantao.zhang@xxxxxxxxx
> > 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
> 

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