Re: [Xen-devel] [RFC PATCH 09/12] libxl: Xen Platform device support for Q35

[Alexey G <x1917x@xxxxxxxxx>]

On Tue, 13 Mar 2018 04:33:54 +1000
Alexey Gerasimenko <x1917x@xxxxxxxxx> wrote:

>Current Xen/QEMU method to control Xen Platform device is a bit odd --
>changing 'xen_platform_device' option value actually modifies QEMU
>emulated machine type, namely xenfv <--> pc.
>
>In order to avoid multiplying machine types, use the new way to control
>Xen Platform device for QEMU -- xen-platform-dev property. To maintain
>backward compatibility with existing Xen/QEMU setups, this is only
>applicable to q35 machine currently. i440 emulation uses the old method
>(xenfv/pc machine) to control Xen Platform device, this may be changed
>later to xen-platform-dev property as well.
>
>Signed-off-by: Alexey Gerasimenko <x1917x@xxxxxxxxx>
>---
> tools/libxl/libxl_dm.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
>diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
>index 7b531050c7..586035aa73 100644
>--- a/tools/libxl/libxl_dm.c
>+++ b/tools/libxl/libxl_dm.c
>@@ -1444,7 +1444,11 @@ static int
>libxl__build_device_model_args_new(libxl__gc *gc,
>         break;
>     case LIBXL_DOMAIN_TYPE_HVM:
>         if (b_info->device_model_machine ==
> LIBXL_DEVICE_MODEL_MACHINE_Q35) {
>-            machinearg = libxl__sprintf(gc, "q35,accel=xen");
>+            if (!libxl_defbool_val(b_info->u.hvm.xen_platform_pci)) {
>+                machinearg = libxl__sprintf(gc, "q35,accel=xen");
>+            } else {
>+                machinearg = libxl__sprintf(gc,
>"q35,accel=xen,xen-platform-dev=on");
>+            }
>         } else {
>             if (!libxl_defbool_val(b_info->u.hvm.xen_platform_pci)) {
>                 /* Switching here to the machine "pc" which does not
> add

Regarding this one -- QEMU maintainers suggested that supplying '-device
xen-platform' directly should be a better approach than a machine
property, so this patch is kinda obsolete.

Right now "xenfv" machine usage for qemu-xen seems to be limited to
controlling the Xen platform device and applying the HVM_MAX_VCPUS
value to maxcpus + minor changes related to IGD passthrough. Both
should be applicable for a "pc,accel=xen" machine as well I think, which
in fact currently lacks the HVM_MAX_VCPUS check for some reason.

Adding a distinct method to control Xen platform device for the q35
machine suggests to propagate the same approach to i440 machine types,
but... it depends on who else can use xenfv for qemu-xen (not to be
confused with xenfv usage on qemu-traditional).

Is there any other toolstacks/code which use xenfv machine solely to
turn on/off Xen platform device?

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