Re: Discrepancy between mmap call on DPDK/libvduse and rust vm-memory crate

["Michael S. Tsirkin" <mst@redhat.com>]

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.


> I can replicate it on the bridge for sure.
> 
> Now I send the VhostUserMemoryRegion to the vhost-user application.
> The struct has these members:
> struct VhostUserMemoryRegion {
>     uint64_t guest_phys_addr;
>     uint64_t memory_size;
>     uint64_t userspace_addr;
>     uint64_t mmap_offset;
> };
> 
> So I can send the offset to the vhost-user device. I can check that
> dpdk-l2fwd uses the same trick of adding offset to the size of the
> mapping region [1], at
> lib/vhost/vhost_user.c:vhost_user_mmap_region():
> 
> mmap_size = region->size + mmap_offset;
> mmap_addr = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
>             MAP_SHARED | populate, region->fd, 0);
> 
> So mmap is called with offset == 0 and everybody is happy.
> 
> Now I'm moving to virtiofsd, and vm-memory crate in particular. And it
> performs the mmap without the size += offset trick, at
> MmapRegionBuilder<B>:build() [2].
> 
> I can try to apply the offset + size trick in my bridge but I don't
> think it is the right solution. At first glance, the right solution is
> to mmap with the offset as vm-memory crate do. But having libvduse and
> DPDK apply the same trick sounds to me like it is a known limitation /
> workaround I don't know about. What is the history of this? Can VDUSE
> problem (if any) be solved? Am I missing something?
> 
> Thanks!
> 
> [1] https://github.com/DPDK/dpdk/blob/e2e546ab5bf5e024986ccb5310ab43982f3bb40c/lib/vhost/vhost_user.c#L1305
> [2] https://github.com/rust-vmm/vm-memory/blob/main/src/mmap_unix.rs#L128