On Sun, Apr 14, 2024 at 5:02 PM Michael S. Tsirkin <m...@redhat.com> wrote: > > On Fri, Apr 12, 2024 at 12:15:40PM +0200, Eugenio Perez Martin wrote: > > Hi! > > > > I'm building a bridge to expose vhost-user devices through VDUSE. The > > code is still immature but I'm able to forward packets using > > dpdk-l2fwd through VDUSE to VM. I'm now developing exposing virtiofsd, > > but I've hit an error I'd like to discuss. > > > > VDUSE devices can get all the memory regions the driver is using by > > VDUSE_IOTLB_GET_FD ioctl. It returns a file descriptor with a memory > > region associated that can be mapped with mmap, and an information > > entry about the map it contains: > > * Start and end addresses from the driver POV > > * Offset within the mmaped region of these start and end > > * Device permissions over that region. > > > > [start=0xc3000][last=0xe7fff][offset=0xc3000][perm=1] > > > > Now when I try to map it, it is impossible for the userspace device to > > call mmap with any offset different than 0. > > How exactly did you allocate memory? hugetlbfs? > > > So the "straightforward" > > mmap with size = entry.last-entry.start and offset = entry.offset does > > not work. I don't know if this is a limitation of Linux or VDUSE. > > > > Checking QEMU's > > subprojects/libvduse/libvduse.c:vduse_iova_add_region() I see it > > handles the offset by adding it up to the size, instead of using it > > directly as a parameter in the mmap: > > > > void *mmap_addr = mmap(0, size + offset, prot, MAP_SHARED, fd, 0); > > > CC Xie Yongji who wrote this code, too. >
The mmap() with hugetlb would fail if the offset into the file is not aligned to the huge page size. So libvhost-user did something like this. But I think VDUSE doesn't have this problem. So it's fine to directly use the offset as a parameter in the mmap(2) here. Thanks, Yongji
