Aaron Lindsay <aa...@os.amperecomputing.com> writes:
> Hello, > > I've been doing some more work with plugins and found something I didn't > expect with regards to address translation. > > If I call (inside a memory callback): > > `uint64_t pa = qemu_plugin_hwaddr_device_offset(hwaddr);` > > I see that `pa` takes the value 0xe0e58760. If, however, I plumb > `cpu_get_phys_page_debug` through to the plugin interface and call it > like: > > `pa = cpu_get_phys_page_debug(current_cpu, va);` > > I see it takes the value 0x120e58760. > > I notice that 0x120e58760-0xe0e58760 is exactly one gigabyte, which is > also the offset of the beginning of RAM for the 'virt' AArch64 machine > I'm using. Furthermore, I see the name of the plugin function includes > "device_offset", so perhaps this discrepancy is by design. However, it > seems awkward to not be able to get a true physical address. It certainly is by design. The comment for the helper states: /* * The following additional queries can be run on the hwaddr structure * to return information about it. For non-IO accesses the device * offset will be into the appropriate block of RAM. */ > I've done some digging and found that inside `qemu_ram_addr_from_host` > (called by `qemu_plugin_hwaddr_device_offset`), `block->mr->addr` > appears to hold the offset of the beginning of RAM. > > Do you think it would be reasonable to modify > `qemu_plugin_hwaddr_device_offset` to add the beginning of the RAM block > or otherwise return the true physical address (or at least expose a way > to find the beginning of it through the plugin interface)? Well the problem here is what is the address map? For example if you have a secure block of RAM you might have two physical addresses which are the same. That said with the current qemu_plugin_hwaddr_device_name helper both will get reported as "RAM" so maybe it's not that helpful yet. I also worry about what happens if devices get moved around. Do you end up with aliasing of address space have a remap of the HW. That said I think we could add an additional helper to translate a hwaddr to a global address space address. I'm open to suggestions of the best way to structure this. > > Thanks! > > -Aaron -- Alex Bennée
