mmMC_VM_FB_LOCATION_TOP
WREG32_SOC15(GC, 0, mmMC_VM_FB_LOCATION_TOP, 0);
WREG32_SOC15(MMHUB, 0, mmMC_VM_FB_LOCATION_TOP,
u64 top = RREG32_SOC15(MMHUB, 0, mmMC_VM_FB_LOCATION_TOP);