On Fri, May 26, 2017 at 10:24:38AM +0800, Haozhong Zhang wrote:
> file_ram_alloc() currently maps the backend file via mmap to a virtual
> address aligned to the value returned by qemu_fd_getpagesize(). When a
> DAX device (e.g. /dev/dax0.0) is used as the backend file, its kernel
> mmap implementation may require an alignment larger than what
> qemu_fd_get_pagesize() returns (e.g. 2MB vs. 4KB), and mmap may fail.
> 
> This commit adds an attribute 'align' to hostmem-file, so that users
> can specify a proper alignment that satisfies the kernel requirement.
> 
> If 'align' is not specified or is 0, the value returned by
> qemu_fd_get_pagesize() will be used as before.

How are users supposed to determine alignment requirements?