[Qemu-devel] Question about the pc.bios mapping on x86

[Maxime Villard]

Hi,
I am developing Qemu support for an accelerator, and I'm facing the following
situation:

The accelerator has a MemoryListener, with a region_add function. Qemu calls
region_add a certain number of times. At one point it wants to map pc.bios,
but the HVA it wants pc.bios mapped at happens to be already occupied by a
mapping done previously.

Typically, a previous call to region_add created a mapping entry with
        [HVA=0x...A2F000 GPA=0xFFFC0000 SIZE=0x40000]
and now Qemu calls region_add to create the pc.bios entry with
        [HVA=0x...A4F000 GPA=0x000E0000 SIZE=0x20000]
As you can see the higher half of the previous HVA mapping collides.

My question is the following: what is the expected behavior here? Is it to
munmap the HVA that collides and remap it to GPA=0xE0000? Is it to duplicate
the content pointed to by the colliding HVA into the new GPA?

I have tested a few combinations, but I can't seem to find the right one, so
I prefer to ask directly to clear the confusion.

Sorry if this is a stupid question, but I didn't find a clear explanation
of the expected behavior.

Thanks