Re: [PATCH v3 18/26] DAX/unmap: virtiofsd: Add VHOST_USER_SLAVE_FS_IO

[Stefan Hajnoczi]

On Wed, Apr 28, 2021 at 12:00:52PM +0100, Dr. David Alan Gilbert (git) wrote:
> @@ -220,6 +222,99 @@ uint64_t vhost_user_fs_slave_unmap(struct vhost_dev 
> *dev, int message_size,
>      return (uint64_t)res;
>  }
>  
> +uint64_t vhost_user_fs_slave_io(struct vhost_dev *dev, int message_size,
> +                                VhostUserFSSlaveMsg *sm, int fd)
> +{
> +    VHostUserFS *fs = (VHostUserFS *)object_dynamic_cast(OBJECT(dev->vdev),
> +                          TYPE_VHOST_USER_FS);
> +    if (!fs) {
> +        error_report("%s: Bad fs ptr", __func__);
> +        return (uint64_t)-1;
> +    }
> +    if (!check_slave_message_entries(sm, message_size)) {
> +        return (uint64_t)-1;
> +    }

These early error returns don't close(fd).

> +
> +    unsigned int i;
> +    int res = 0;
> +    size_t done = 0;
> +
> +    if (fd < 0) {
> +        error_report("Bad fd for io");
> +        return (uint64_t)-1;
> +    }
> +
> +    for (i = 0; i < sm->count && !res; i++) {
> +        VhostUserFSSlaveMsgEntry *e = &sm->entries[i];
> +        if (e->len == 0) {
> +            continue;
> +        }
> +
> +        size_t len = e->len;
> +        uint64_t fd_offset = e->fd_offset;
> +        hwaddr gpa = e->c_offset;
> +
> +        while (len && !res) {
> +            hwaddr xlat, xlat_len;
> +            bool is_write = e->flags & VHOST_USER_FS_FLAG_MAP_W;
> +            MemoryRegion *mr = address_space_translate(dev->vdev->dma_as, 
> gpa,
> +                                                       &xlat, &xlat_len,
> +                                                       is_write,
> +                                                       
> MEMTXATTRS_UNSPECIFIED);
> +            if (!mr || !xlat_len) {
> +                error_report("No guest region found for 0x%" HWADDR_PRIx, 
> gpa);
> +                res = -EFAULT;
> +                break;
> +            }
> +
> +            trace_vhost_user_fs_slave_io_loop(mr->name,
> +                                          (uint64_t)xlat,
> +                                          memory_region_is_ram(mr),
> +                                          memory_region_is_romd(mr),
> +                                          (size_t)xlat_len);
> +
> +            void *hostptr = qemu_map_ram_ptr(mr->ram_block,
> +                                             xlat);
> +            ssize_t transferred;

What happens when the MemoryRegion is not backed by RAM?

> +            if (e->flags & VHOST_USER_FS_FLAG_MAP_R) {
> +                /* Read from file into RAM */
> +                if (mr->readonly) {
> +                    res = -EFAULT;
> +                    break;
> +                }
> +                transferred = pread(fd, hostptr, xlat_len, fd_offset);
> +            } else if (e->flags & VHOST_USER_FS_FLAG_MAP_W) {
> +                /* Write into file from RAM */
> +                transferred = pwrite(fd, hostptr, xlat_len, fd_offset);
> +            } else {
> +                transferred = EINVAL;

I don't see how this is handled below by the error checking code. Should
this be:

  errno = EINVAL;
  transferred = -1;

?

> +            }
> +
> +            trace_vhost_user_fs_slave_io_loop_res(transferred);
> +            if (transferred < 0) {
> +                res = -errno;
> +                break;
> +            }
> +            if (!transferred) {
> +                /* EOF */
> +                break;
> +            }
> +
> +            done += transferred;
> +            fd_offset += transferred;
> +            gpa += transferred;
> +            len -= transferred;
> +        }
> +    }
> +    close(fd);
> +
> +    trace_vhost_user_fs_slave_io_exit(res, done);
> +    if (res < 0) {
> +        return (uint64_t)res;
> +    }
> +    return (uint64_t)done;
> +}

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