From: Alex Williamson Sent: Wednesday, February 17, 2016 5:39 AM
QEMU provides two fw_cfg files to support IGD. The first holds the OpRegion data which holds the Video BIOS Table (VBT). This needs to be copied into reserved memory and the address stored in the ASL Storage register of the device at 0xFC offset in PCI config space. The OpRegion is generally 8KB. This file is named "etc/igd-opregion".
The second file tells us the required size of the stolen memory space for the device. This is a dummy file, it has no backing so we only allocate the space without copying anything into it. This space requires 1MB alignment and is generally either 1MB or 2MB, depending on the hardware config. If the user has opted in QEMU to expose additional stolen memory beyond the GTT (GGMS), the GMS may add an additional 32MB to 512MB. The base address of the reserved memory allocated for this is written back to the Base Data of Stolen Memory register (BDSM) at PCI config offset 0x5C on the device. This file is named "etc/igd-bdsm".
What would happen if guest tries to access this range while there is no actual memory behind? Isn't it more clear to hide stolen memory at all instead of reporting a dummy range?