Re: [PATCH] QEMU as non-root and PCI passthrough do not mix

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2016-01-12 at 16:52 +0000, Stefano Stabellini wrote:
> 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
> hotplug.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> 
> 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 *gc,
>              break;
>          }
>  
> +        /* 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;
> +        }

What if b_info->device_model_user is NULL or == "root"? Doesn't this warn
even then?

Conversely if it is != root and num_pcidevs > 0 then it ought to error out,
since running as root when the config explicitly says otherwise would be
wrong I think.

> +
>          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");

Shouldn't there be an if here, and/or an error return?

> +
>      rc = libxl__device_pci_add(gc, domid, pcidev, 0);
>      libxl__ao_complete(egc, ao, rc);
>      return AO_INPROGRESS;

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