Re: [PATCH v6] xen/pt: reserve PCI slot 2 for Intel igd-passthru

[Anthony PERARD]

On Sun, Jan 01, 2023 at 06:52:03PM -0500, Chuck Zmudzinski wrote:
> Intel specifies that the Intel IGD must occupy slot 2 on the PCI bus,
> as noted in docs/igd-assign.txt in the Qemu source code.
> 
> Currently, when the xl toolstack is used to configure a Xen HVM guest with
> Intel IGD passthrough to the guest with the Qemu upstream device model,
> a Qemu emulated PCI device will occupy slot 2 and the Intel IGD will occupy
> a different slot. This problem often prevents the guest from booting.
> 
> The only available workaround is not good: Configure Xen HVM guests to use
> the old and no longer maintained Qemu traditional device model available
> from xenbits.xen.org which does reserve slot 2 for the Intel IGD.
> 
> To implement this feature in the Qemu upstream device model for Xen HVM
> guests, introduce the following new functions, types, and macros:
> 
> * XEN_PT_DEVICE_CLASS declaration, based on the existing TYPE_XEN_PT_DEVICE
> * XEN_PT_DEVICE_GET_CLASS macro helper function for XEN_PT_DEVICE_CLASS
> * typedef XenPTQdevRealize function pointer
> * XEN_PCI_IGD_SLOT_MASK, the value of slot_reserved_mask to reserve slot 2
> * xen_igd_reserve_slot and xen_igd_clear_slot functions
> 
> The new xen_igd_reserve_slot function uses the existing slot_reserved_mask
> member of PCIBus to reserve PCI slot 2 for Xen HVM guests configured using
> the xl toolstack with the gfx_passthru option enabled, which sets the
> igd-passthru=on option to Qemu for the Xen HVM machine type.
> 
> The new xen_igd_reserve_slot function also needs to be implemented in
> hw/xen/xen_pt_stub.c to prevent FTBFS during the link stage for the case
> when Qemu is configured with --enable-xen and --disable-xen-pci-passthrough,
> in which case it does nothing.
> 
> The new xen_igd_clear_slot function overrides qdev->realize of the parent
> PCI device class to enable the Intel IGD to occupy slot 2 on the PCI bus
> since slot 2 was reserved by xen_igd_reserve_slot when the PCI bus was
> created in hw/i386/pc_piix.c for the case when igd-passthru=on.
> 
> Move the call to xen_host_pci_device_get, and the associated error
> handling, from xen_pt_realize to the new xen_igd_clear_slot function to
> initialize the device class and vendor values which enables the checks for
> the Intel IGD to succeed. The verification that the host device is an
> Intel IGD to be passed through is done by checking the domain, bus, slot,
> and function values as well as by checking that gfx_passthru is enabled,
> the device class is VGA, and the device vendor in Intel.
> 
> Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>


This patch looks good enough. It only changes the "xenfv" machine so it
doesn't prevent a proper fix to be done in the toolstack libxl.

The change in xen_pci_passthrough_class_init() to try to run some code
before pci_qdev_realize() could potentially break in the future due to
been uncommon but hopefully that will be ok.

So if no work to fix libxl appear soon, I'm ok with this patch:

Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

Thanks,

-- 
Anthony PERARD