Re: PCI Device Subtree Change from Traditional to Upstream

[Anthony PERARD <anthony.perard@citrix.com>]

On Wed, Dec 20, 2017 at 11:40:03AM -0600, Kevin Stange wrote:
> Hi,
> 
> I've been working on transitioning a number of Windows guests under HVM
> from using QEMU traditional to QEMU upstream as is recommended in the
> documentation.  When I move these guests, the PCI subtree for Xen
> devices changes and Windows creates a totally new copy of each device.
> Windows tracks down the storage without issue, but it treats the new
> instance of the NIC driver as a new device and clears the network
> configuration even though the MAC address is unchanged.  Manually
> booting the guest back on the traditional device model reactivates the
> original PCI subtree and the old network configuration with it.
> 
> The only thing that I have been able to find that's substantially
> different comparing the device trees is that the device instance ID
> values differ on the parent Xen PCI device:
> 
> PCI\VEN_5853&DEV_0001&SUBSYS_00015853&REV_01\3&267A616A&3&18
> 
> PCI\VEN_5853&DEV_0001&SUBSYS_00015853&REV_01\3&267A616A&3&10
> 
> Besides actually setting the guest to boot using QEMU traditional, is
> there a way to convince Windows to treat these devices as the same?  A
> patch-based solution would be acceptable to me if there is one, but I
> don't understand the code well enough to create my own solution.

Hi Kevin,

I've got a patch to QEMU that seems to do the trick:

From: Anthony PERARD <anthony.perard@citrix.com>
Subject: [PATCH] xen-platform: Hardcode PCI slot to 3

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
---
 hw/i386/pc_piix.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 5e47528993..93e3a9a916 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -405,7 +405,7 @@ static void pc_xen_hvm_init(MachineState *machine)
 
     bus = pci_find_primary_bus();
     if (bus != NULL) {
-        pci_create_simple(bus, -1, "xen-platform");
+        pci_create_simple(bus, PCI_DEVFN(3, 0), "xen-platform");
     }
 }
 #endif


The same thing could be done by libxl, by providing specific command
line options to qemu. (I think that could even be done via a different
config file for the guest.)

Regards,

-- 
Anthony PERARD

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