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[Xen-devel] [PATCH] QEMU as non-root and PCI passthrough do not mix

PCI passthrough cannot work if QEMU is run as a non-root process today,
as QEMU needs to open /dev/mem to mmap the MSI-X table of the device and
read/write relevant nodes on sysfs.

Update the docs to reflect that.

Run QEMU as root and print a warning if at least one PCI device has been
assigned to the guest at domain creation. Print a debug message on pci

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>

diff --git a/docs/misc/qemu-deprivilege.txt b/docs/misc/qemu-deprivilege.txt
index dde74ab..cf52547 100644
--- a/docs/misc/qemu-deprivilege.txt
+++ b/docs/misc/qemu-deprivilege.txt
@@ -29,3 +29,13 @@ adduser --no-create-home --system xen-qemuuser-shared
 3) root
 As a last resort, libxl will start QEMU as root.
+Please note that QEMU will still be run as root when PCI devices are
+assigned to the virtual machine (if you specified pci=["$PCI_BDF"] in
+your VM config file, where $PCI_BDF is the PCI BDF of the device you
+want to assign). If you want to hotplug a PCI device sometime after the
+VM has started, you need to make sure that the QEMU instance of that VM
+has root privileges (for example by not specifying either
+xen-qemuuser-shared or xen-qemuuser-domid$domid, or by giving root
+privileges to xen-qemuuser-domid$domid).
diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index 0aaefd9..6b98750 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -1254,6 +1254,12 @@ static int libxl__build_device_model_args_new(libxl__gc 
+        /* Do not run QEMU as non-root if PCI devices are assigned */
+        if (guest_config->num_pcidevs > 0) {
+            LOG(WARN, "Cannot run QEMU as non-root when PCI devices are being 
assigned to the guest VM");
+            goto end_search;
+        }
         if (b_info->device_model_user) {
             user = b_info->device_model_user;
             goto end_search;
diff --git a/tools/libxl/libxl_pci.c b/tools/libxl/libxl_pci.c
index dc10cb7..04d0dd4 100644
--- a/tools/libxl/libxl_pci.c
+++ b/tools/libxl/libxl_pci.c
@@ -1176,6 +1176,9 @@ int libxl_device_pci_add(libxl_ctx *ctx, uint32_t domid,
     AO_CREATE(ctx, domid, ao_how);
     int rc;
+    LOG(DEBUG, "QEMU needs to be run as root for PCI passthrough to work");
     rc = libxl__device_pci_add(gc, domid, pcidev, 0);
     libxl__ao_complete(egc, ao, rc);
     return AO_INPROGRESS;

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