On 9/22/2026 5:56 PM, Cédric Le Goater wrote:
Hello Yuan
On 9/22/26 07:36, Yuan Wang wrote:
Meteor Lake and Arrow Lake GOP can read BAR0 MMIO offset 0x138914 to
detect whether direct framebuffer access via DSM is available.
In a VFIO passthrough VM, the guest cannot access the host DSM memory
region via such a pointer. If the guest GOP enables that path, it may
hand off a DSM-based framebuffer address that the guest cannot actually
use.
Intercept reads from the detection register and return 0 so the guest
does not enable the DSM-based path and instead keeps using the standard
BAR-based framebuffer address.
Suggested-by: Tomita Moeko <[email protected]>
Reviewed-by: Tomita Moeko <[email protected]>
Reviewed-by: Bosheng Xue <[email protected]>
Reviewed-by: Junjie Cao <[email protected]>
Reviewed-by: Cédric Le Goater <[email protected]>
Signed-off-by: Yuan Wang <[email protected]>
---
Changes:
v4:
- Address review comments from Tomita Moeko (update macro/variable
names and comments).
- Pick up Reviewed-by tags from Tomita Moeko and Cédric Le Goater.
v3:
- Emulated register 0x138914 to return 0x0, which signals the guest
GOP driver to fall back to BAR-based mapping since DSM is
unavailable in the VM.
v2:
- Resending because the v1 patch was sent with an incorrect future
system timestamp due to an unsynchronized local clock. No code
changes.
Notes:
MTL/ARL device IDs remain in igd_gen().
This is required because vfio_probe_igd_bar0_quirk() checks the
generation before installing BAR0 quirks, and the 0x138914 quirk
is added afterward.
Meanwhile, we also plan to remove the bdsm-size check in the OVMF
offline patches.
---
hw/vfio/igd.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 46 insertions(+), 3 deletions(-)
This change does not apply cleanly on :
https://github.com/legoater/qemu vfio-next
because of :
https://lore.kernel.org/qemu-devel/[email protected]/
Could you please resend on top ?
Sure! I have rebased the patch on top of your vfio-next branch and sent out v5.
Please kindly review the v5.
Thanks,
Yuan
