Re: [XEN PATCH v2 0/3] Configure qemu upstream correctly by default for igd-passthru

[Chuck Zmudzinski]

On 1/25/23 6:19 PM, Chuck Zmudzinski wrote:
> On 1/25/2023 6:37 AM, Anthony PERARD wrote:
>> On Tue, Jan 10, 2023 at 02:32:01AM -0500, Chuck Zmudzinski wrote:
>> > I call attention to the commit message of the first patch which points
>> > out that using the "pc" machine and adding the xen platform device on
>> > the qemu upstream command line is not functionally equivalent to using
>> > the "xenfv" machine which automatically adds the xen platform device
>> > earlier in the guest creation process. As a result, there is a noticeable
>> > reduction in the performance of the guest during startup with the "pc"
>> > machne type even if the xen platform device is added via the qemu
>> > command line options, although eventually both Linux and Windows guests
>> > perform equally well once the guest operating system is fully loaded.
>>
>> There shouldn't be a difference between "xenfv" machine or using the
>> "pc" machine while adding the "xen-platform" device, at least with
>> regards to access to disk or network.
>>
>> The first patch of the series is using the "pc" machine without any
>> "xen-platform" device, so we can't compare startup performance based on
>> that.
>>
>> > Specifically, startup time is longer and neither the grub vga drivers
>> > nor the windows vga drivers in early startup perform as well when the
>> > xen platform device is added via the qemu command line instead of being
>> > added immediately after the other emulated i440fx pci devices when the
>> > "xenfv" machine type is used.
>>
>> The "xen-platform" device is mostly an hint to a guest that they can use
>> pv-disk and pv-network devices. I don't think it would change anything
>> with regards to graphics.
>>
>> > For example, when using the "pc" machine, which adds the xen platform
>> > device using a command line option, the Linux guest could not display
>> > the grub boot menu at the native resolution of the monitor, but with the
>> > "xenfv" machine, the grub menu is displayed at the full 1920x1080
>> > native resolution of the monitor for testing. So improved startup
>> > performance is an advantage for the patch for qemu.
>>
>> I've just found out that when doing IGD passthrough, both machine
>> "xenfv" and "pc" are much more different than I though ... :-(
>> pc_xen_hvm_init_pci() in QEMU changes the pci-host device, which in
>> turns copy some informations from the real host bridge.
>> I guess this new host bridge help when the firmware setup the graphic
>> for grub.

Yes, it is needed - see below for the very simple patch to Qemu
upstream that fixes it for the "pc" machine!

> 
> I am surprised it works at all with the "pc" machine, that is, without the
> TYPE_IGD_PASSTHROUGH_I440FX_PCI_DEVICE that is used in the "xenfv"
> machine. This only seems to affect the legacy grub vga driver and the legacy
> Windows vga driver during early boot. Still, I much prefer keeping the "xenfv"
> machine for Intel IGD than this workaround of patching libxl to use the "pc"
> machine.
> 
>>
>> > I also call attention to the last point of the commit message of the
>> > second patch and the comments for reviewers section of the second patch.
>> > This approach, as opposed to fixing this in qemu upstream, makes
>> > maintaining the code in libxl__build_device_model_args_new more
>> > difficult and therefore increases the chances of problems caused by
>> > coding errors and typos for users of libxl. So that is another advantage
>> > of the patch for qemu.
>>
>> We would just needs to use a different approach in libxl when generating
>> the command line. We could probably avoid duplications.

I was thinking we could also either write a test to verify the correctness
of the second patch to ensure it generates the correct Qemu command line
or take the time to verify the second patch's accuracy before committing it.

>> I was hopping to
>> have patch series for libxl that would change the machine used to start
>> using "pc" instead of "xenfv" for all configurations, but based on the
>> point above (IGD specific change to "xenfv"), then I guess we can't
>> really do anything from libxl to fix IGD passthrough.
> 
> We could switch to the "pc" machine, but we would need to patch
> qemu also so the "pc" machine uses the special device the "xenfv"
> machine uses (TYPE_IGD_PASSTHROUGH_I440FX_PCI_DEVICE).
> ...

I just tested a very simple patch to Qemu upstream to fix the
difference between the upstream Qemu "pc" machine and the upstream
Qemu "xenfv" machine:

--- a/hw/i386/pc_piix.c 2023-01-28 13:22:15.714595514 -0500
+++ b/hw/i386/pc_piix.c 2023-01-29 18:08:34.668491593 -0500
@@ -434,6 +434,8 @@
             compat(machine); \
         } \
         pc_init1(machine, TYPE_I440FX_PCI_HOST_BRIDGE, \
+                 xen_igd_gfx_pt_enabled() ? \
+                 TYPE_IGD_PASSTHROUGH_I440FX_PCI_DEVICE : \
                  TYPE_I440FX_PCI_DEVICE); \
     } \
     DEFINE_PC_MACHINE(suffix, name, pc_init_##suffix, optionfn)
----- snip -------

With this simple two-line patch to upstream Qemu, we can use the "pc"
machine without any regressions such as the startup performance
degradation I observed without this small patch to fix the "pc" machine
with igd passthru!

The "pc" machine maintainers for upstream Qemu would need to accept
this small patch to Qemu upstream. They might prefer this to the
other Qemu patch that reserves slot 2 for the Qemu upstream "xenfv"
machine when the guest is configured for igd passthru.

>>
>> ...
>>
>> So overall, unfortunately the "pc" machine in QEMU isn't suitable to do
>> IGD passthrough as the "xenfv" machine has already some workaround to
>> make IGD work and just need some more.

Well, the little patch to upstream shown above fixes the "pc" machine
with IGD so maybe this approach of patching libxl to use the "pc" machine
will be a viable fix for the IGD.

>>
>> I've seen that the patch for QEMU is now reviewed, so I look at having
>> it merged soonish.
>>
>> Thanks,
>>
> 

I just added the bit of information above to help you decide which
approach to use to improve the support for the igd passthru feature
with Xen and Qemu upstream. I think the test of the small patch to
Qemu to fix the "pc" machine with igd passthru makes this patch to
libxl a viable alternative to the other patch to Qemu upstream that
reserves slot 2 when using the "xenfv" machine and igd passthru.

Thanks,

Chuck