* jack.chen (zhun...@gmail.com) wrote:
> hello,I am confused when I read vhost-user source code in qemu.I know
> vhost-user app shared memory with qemu by mmap,but why it can use fd which
> is belong to qemu?
> relative code:
> qemu code in function vhost_user_set_mem_table
> fd = memory_region_get_fd(mr);
> if (fd > 0) {
> msg.payload.memory.regions[fd_num].userspace_addr =
> reg->userspace_addr;
> msg.payload.memory.regions[fd_num].memory_size =
> reg->memory_size;
> msg.payload.memory.regions[fd_num].guest_phys_addr =
> reg->guest_phys_addr;
> msg.payload.memory.regions[fd_num].mmap_offset = offset;
> assert(fd_num < VHOST_MEMORY_MAX_NREGIONS);
> fds[fd_num++] = fd;
> }
>
> ……
> DPDK code in vhost_user_set_mem_table
>
> mmap_addr = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
> MAP_SHARED | MAP_POPULATE, fd, 0);
> ……
>
> thanks a lot!
Because that's how the dpdk/vhost-user binary knows what to mmap;
each fd corresponds to the backing file of the memory area that's being
shared. This way the dpdk/vhost doesn't need to open those files itself
or try and match the exact memory configuration of qemu; QEMU just gives
it the exact thing it needs to mmap - which is just the fd and offsets.
Dave
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
