Re: [Qemu-devel] [PATCH V14 0/3] Virtual Machine Generation ID

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 15, 2015 at 03:59:09PM +0200, Igor Mammedov wrote:
> On Wed, 15 Apr 2015 12:38:57 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Tue, Mar 03, 2015 at 05:18:12PM +0100, Igor Mammedov wrote:
> > > Changes since v13:
> > >  * fix comment style to /*... */ in testcase
> > >  * make BAR TARGET_PAGE_SIZE as required by spec
> > >  * make BAR prefetchable, spec also says that it shouldn't be
> > >    marked as non cached
> > >  * ACPI part
> > >     * merge separate VGID device with PCI device description
> > >     * mark device as not shown in UI,
> > >       it hides "VM Generation ID" device from device manager
> > >       and leaves only "PCI Standard RAM Controller" device there
> > 
> > In an offline chat, Yan (Cc) mentioned that with windows guests,
> > PCI rebalancing can move BAR values around.
> > Yan, could you comment on-list please?
> > Is there a way to force this, for testing?
> > 
> > ACPI spec mentions this:
> > If a platform uses a PCI BAR Target operation region, an ACPI OS will
> > not load a native device driver for the associated PCI function. For
> > example, if any of the BARs in a PCI function are associated with a PCI
> > BAR Target operation region, then the OS will assume that the PCI
> > function is to be entirely under the control of the ACPI BIOS. No driver
> > will be loaded. Thus, a PCI function can be used as a platform
> > controller for some task (hot-plug PCI, and so on) that the ACPI BIOS
> > performs.
> It seems that WS2012R2 doesn't honor this part of spec,
> it still tries to find matching driver and load it.

Interesting. Looking at msdn:
https://msdn.microsoft.com/en-us/library/windows/hardware/ff563877(v=vs.85).aspx
Looks like what windows does to rebalance is query device for
rebalancing support, stop driver, rebalance, and restart driver.

If device has no driver, it seems likely windows will assume
it's safe to rebalance the resources.

There's also a very old document:
http://www.microsoft.com/whdc/connect/pci/PCI-rsc.mspx
which says it's possible to implement _DSM method to deny rebalance
requests.
Alternatively, we could put this device behind a pci extender, e.g. as
implemented by Marcel. The resources would then be limited by the _CRS
or the root.

> > 
> > This might also avoid driver prompt from windows?
> it isn't.