When pass-thru GPU to pre-launched Linux guest,
need to pass GPU OpRegion to the guest.
Here's the detailed steps:
1. reserve a memory region in ve820 table for GPU OpRegion
2. build EPT mapping for GPU OpRegion to pass-thru OpRegion to guest
3. emulate the pci config register for OpRegion
For the third step, here's detailed description:
The address of OpRegion locates on PCI config space offset 0xFC,
Normal Linux guest won't write this register,
so we can regard this register as read-only.
When guest reads this register, return the emulated value.
When guest writes this register, ignore the operation.
Tracked-On: #6387
Signed-off-by: Liu,Junming <junming.liu@intel.com>
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