While the physical BAR is present on the device, it is not being used on the host system. The access to the device memory region on the host occur through the C2C interconnect link (not the PCIe) and is present for access as a separate memory region in the physical address space on the host. The variant driver queries this range from the host ACPI DSD tables. Now, this device memory region on the host is exposed as a device BAR in the VM. So the device BAR in the VM is actually mapped to the device memory region in the physical address space (and not to the physical BAR) on the host. The config space accesses to the device however, are still going to the physical BAR on the host. > Does this BAR2 size match the size we're reporting for the region? Now > I'm confused why we need to intercept the BAR2 region info if there's > physically a real BAR behind it. Thanks, Yes, it does match the size being reported through region info. But the region info ioctl is still intercepted to provide additional cap to establish the sparse mapping. Why we do sparse mapping? The actual device memory size is not power-of-2 aligned (a requirement for a BAR). So we roundup to the next power-of-2 value and report the size as such. Then we utilize sparse mapping to show only the actual size of the device memory as mappable.
