Re: [PATCH 00/10] vfio/igd: Remove legacy mode

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 25 Feb 2025 02:29:17 +0800
Tomita Moeko <tomitamoeko@gmail.com> wrote:

> This patchset removes some legacy checks and converts the legacy mode
> implicitly enabled by BDF 00:02.0 into x-igd-* options, including:
> * Removing PCI ROM BAR and VGA IO/MMIO range check before applying quirk
> * Using unified x-igd-opregion option for OpRegion access.
> * Introducing new x-igd-lpc option for the LPC bridge / Host bridge ID
>   quirk. Currently this is only supported on i440fx.
> * Extending quirk support when IGD is not assigned to BDF 00:02.0
> 
> The first 2 patches of this patchset was taken from a previous one,
> details can be found at:
> https://lore.kernel.org/all/20250124191245.12464-1-tomitamoeko@gmail.com/
> 
> This patchest was mainly tested on Alder Lake UHD770, with Debian 12
> (kernel 6.1), Windows 11 (driver 32.0.101.6458) and Intel GOP driver
> 17.0.1081.
> 
> If the design is good to go, I will update the documentation also.
> 
> A open question is whether the old legacy mode behavior should be kept
> or not. Checking if all the condition of legacy mode were met and
> toggling correspoding options is more complicated then I expected :(
> Any ideas would be appreciated.

I dusted off a working IGD assignment configuration on an i7-7700 Kaby
Lake system with HD630 graphics.  This series breaks it a couple times.

First, while the host system itself may support UEFI (some systems may
not), I had dumped the VGA ROM in CSM mode and run the VM with SeaBIOS.
Therefore it needs VGA support.  This is the recommended configuration
for legacy mode in our current documentation.  Patch 4/ requires that I
add x-vga to continue using this VM.

Then of course since this VM is specifically using legacy mode, patch
10/ requires that I also add both x-igd-opregion and x-igd-lpc.

Therefore I go from a VM that works with IGD assignment in legacy mode,
with no unsupported options:

<domain type="kvm">
    ...
    <type arch="x86_64" machine="pc-i440fx-10.0">hvm</type>
    ...
    <hostdev mode="subsystem" type="pci" managed="yes">
      <source>
        <address domain="0x0000" bus="0x00" slot="0x02" function="0x0"/>
      </source>
      <rom file="/var/lib/libvirt/images/igd-i7-7700.rom"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x0"/>
    </hostdev>

To one that requires multiple unsupported, experimental options:

<domain xmlns:qemu="http://libvirt.org/schemas/domain/qemu/1.0" type="kvm">
    ...
    <type arch="x86_64" machine="pc-i440fx-10.0">hvm</type>
    ...
    <hostdev mode="subsystem" type="pci" managed="yes">
      <source>
        <address domain="0x0000" bus="0x00" slot="0x02" function="0x0"/>
      </source>
      <alias name="ua-igd"/>
      <rom file="/var/lib/libvirt/images/igd-i7-7700.rom"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x0"/>
    </hostdev>
  ...
  <qemu:override>
    <qemu:device alias="ua-igd">
      <qemu:frontend>
        <qemu:property name="x-vga" type="bool" value="true"/>
        <qemu:property name="x-igd-opregion" type="bool" value="true"/>
        <qemu:property name="x-igd-lpc" type="bool" value="true"/>
      </qemu:frontend>
    </qemu:device>
  </qemu:override>

I don't know how we justify this within our defacto support contract
with users.  Legacy mode needs to continue to exist and be
automatically enabled unless we want to figure out a deprecation
process for it and queue this sort of change for sometime in the
future.  Thanks,

Alex