Re: [Qemu-devel] [RFC PATCH v4] fw/pci: Add support for mapping Intel IGD via QEMU

[Tian, Kevin]

> 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?

Thanks
Kevin