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Re: [Qemu-devel] [PATCH v19 3/9] pc: add a Virtual Machine Generation ID


From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH v19 3/9] pc: add a Virtual Machine Generation ID device
Date: Tue, 16 Feb 2016 17:10:13 +0200

On Tue, Feb 16, 2016 at 02:36:49PM +0200, Marcel Apfelbaum wrote:
> >>2. PCI devices with no driver installed are not re-mapped. This can be OK
> >>     from the Windows point of view because Resources Window does not show 
> >> the MMIO range
> >>     for this device.
> >>
> >>     If the other (re-mapped) device is working, is pure luck. Both Memory 
> >> Regions occupy the same range
> >>     and have the same priority.
> >>
> >>We need to think about how to solve this.
> >>One way would be to defer the BAR activation to the guest OS, but I am not 
> >>sure of the consequences.
> >deferring won't solve problem as rebalancing could happen later
> >and make BARs overlap.
> 
> Why not? If we do not activate the BAR in firmware and Windows does not have 
> a driver
> for it, will not activate it at all, right?
> Why would Windows activate the device BAR if it can't use it? At least this 
> is what I hope.
> Any other idea would be appreciated.
> 

I wonder whether this is related to setting PnP in CMOS.
See e.g.  http://oss.sgi.com/LDP/HOWTO/Plug-and-Play-HOWTO-4.html
and https://support.microsoft.com/en-us/kb/321779



> >I've noticed that at startup Windows unmaps and then maps BARs
> >at the same addresses where BIOS've put them before.
> 
> Including devices without a working driver?
> 
> 
> Thanks,
> Marcel
> 
> >
> >>And this does not solve the ivshmem problem.
> >So far the only way to avoid overlapping BARs due to Windows
> >doing rebalancing for driver-less devices is to pin such
> >BARs statically with _CRS in ACPI table but as Michael said
> >it fragments PCI address-space.
> >
> >>
> >>Thanks,
> >>Marcel
> >>
> >>
> >>
> >>
> >>
> >



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