Re: [PATCH 16/24] DAX/unmap: virtiofsd: Add VHOST_USER_SLAVE_FS_IO

[Stefan Hajnoczi]

On Tue, Feb 09, 2021 at 07:02:16PM +0000, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> Define a new slave command 'VHOST_USER_SLAVE_FS_IO' for a
> client to ask qemu to perform a read/write from an fd directly
> to GPA.
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  docs/interop/vhost-user.rst               | 11 +++
>  hw/virtio/trace-events                    |  6 ++
>  hw/virtio/vhost-user-fs.c                 | 84 +++++++++++++++++++++++
>  hw/virtio/vhost-user.c                    |  4 ++
>  include/hw/virtio/vhost-user-fs.h         |  2 +
>  subprojects/libvhost-user/libvhost-user.h |  1 +
>  6 files changed, 108 insertions(+)
> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index 1deedd3407..821712f4a2 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -1452,6 +1452,17 @@ Slave message types
>    multiple chunks can be unmapped in one command.
>    A reply is generated indicating whether unmapping succeeded.
>  
> +``VHOST_USER_SLAVE_FS_IO``
> +  :id: 9
> +  :equivalent ioctl: N/A
> +  :slave payload: fd + n * (offset + address + len)

Please clarify the payload representation. This is not enough for
someone to implement the spec.

> +  :master payload: N/A
> +
> +  Requests that the QEMU performs IO directly from an fd to guest memory

To avoid naming a particular VMM:

s/the QEMU performs IO/IO be performed/

> +  on behalf of the daemon; this is normally for a case where a memory region
> +  isn't visible to the daemon. slave payload has flags which determine
> +  the direction of IO operation.

Please document the payload flags in the spec.

> +
>  .. _reply_ack:
>  
>  VHOST_USER_PROTOCOL_F_REPLY_ACK
> diff --git a/hw/virtio/trace-events b/hw/virtio/trace-events
> index c62727f879..20557a078e 100644
> --- a/hw/virtio/trace-events
> +++ b/hw/virtio/trace-events
> @@ -53,6 +53,12 @@ vhost_vdpa_get_features(void *dev, uint64_t features) 
> "dev: %p features: 0x%"PRI
>  vhost_vdpa_set_owner(void *dev) "dev: %p"
>  vhost_vdpa_vq_get_addr(void *dev, void *vq, uint64_t desc_user_addr, 
> uint64_t avail_user_addr, uint64_t used_user_addr) "dev: %p vq: %p 
> desc_user_addr: 0x%"PRIx64" avail_user_addr: 0x%"PRIx64" used_user_addr: 
> 0x%"PRIx64
>  
> +# vhost-user-fs.c
> +
> +vhost_user_fs_slave_io_loop(const char *name, uint64_t owr, int is_ram, int 
> is_romd, size_t size) "region %s with internal offset 0x%"PRIx64 " ram=%d 
> romd=%d mrs.size=%zd"
> +vhost_user_fs_slave_io_loop_res(ssize_t transferred) "%zd"
> +vhost_user_fs_slave_io_exit(int res, size_t done) "res: %d done: %zd"
> +
>  # virtio.c
>  virtqueue_alloc_element(void *elem, size_t sz, unsigned in_num, unsigned 
> out_num) "elem %p size %zd in_num %u out_num %u"
>  virtqueue_fill(void *vq, const void *elem, unsigned int len, unsigned int 
> idx) "vq %p elem %p len %u idx %u"
> diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
> index 5f2fca4d82..357bc1d04e 100644
> --- a/hw/virtio/vhost-user-fs.c
> +++ b/hw/virtio/vhost-user-fs.c
> @@ -23,6 +23,8 @@
>  #include "hw/virtio/vhost-user-fs.h"
>  #include "monitor/monitor.h"
>  #include "sysemu/sysemu.h"
> +#include "exec/address-spaces.h"
> +#include "trace.h"
>  
>  /*
>   * The powerpc kernel code expects the memory to be accessible during
> @@ -155,6 +157,88 @@ uint64_t vhost_user_fs_slave_unmap(struct vhost_dev *dev,
>      return (uint64_t)res;
>  }
>  
> +uint64_t vhost_user_fs_slave_io(struct vhost_dev *dev, VhostUserFSSlaveMsg 
> *sm,
> +                                int fd)
> +{
> +    VHostUserFS *fs = VHOST_USER_FS(dev->vdev);
> +    if (!fs) {
> +        /* Shouldn't happen - but seen it in error paths */
> +        error_report("Bad fs ptr");
> +        return (uint64_t)-1;
> +    }

Same pointer casting issue as with map/unmap.

> +
> +    unsigned int i;
> +    int res = 0;
> +    size_t done = 0;
> +
> +    if (fd < 0) {
> +        error_report("Bad fd for map");
> +        return (uint64_t)-1;
> +    }
> +
> +    for (i = 0; i < VHOST_USER_FS_SLAVE_ENTRIES && !res; i++) {
> +        if (sm->len[i] == 0) {
> +            continue;
> +        }
> +
> +        size_t len = sm->len[i];
> +        hwaddr gpa = sm->c_offset[i];
> +
> +        while (len && !res) {
> +            MemoryRegionSection mrs = memory_region_find(get_system_memory(),
> +                                                         gpa, len);
> +            size_t mrs_size = (size_t)int128_get64(mrs.size);

If there is a vIOMMU then the vhost-user device backend should be
restricted to just areas of guest RAM that are mapped. I think this can
be achieved by using the vhost-user-fs device's address space instead of
get_system_memory(). For example, virtio_pci_get_dma_as().

> +
> +            if (!mrs_size) {
> +                error_report("No guest region found for 0x%" HWADDR_PRIx, 
> gpa);
> +                res = -EFAULT;
> +                break;
> +            }
> +
> +            trace_vhost_user_fs_slave_io_loop(mrs.mr->name,
> +                                          (uint64_t)mrs.offset_within_region,
> +                                          memory_region_is_ram(mrs.mr),
> +                                          memory_region_is_romd(mrs.mr),
> +                                          (size_t)mrs_size);
> +
> +            void *hostptr = qemu_map_ram_ptr(mrs.mr->ram_block,
> +                                             mrs.offset_within_region);
> +            ssize_t transferred;
> +            if (sm->flags[i] & VHOST_USER_FS_FLAG_MAP_R) {

The flag name is specific to map requests but it's shared with the IO
request. Perhaps rename the flags?

> +                /* Read from file into RAM */
> +                if (mrs.mr->readonly) {
> +                    res = -EFAULT;
> +                    break;
> +                }
> +                transferred = pread(fd, hostptr, mrs_size, sm->fd_offset[i]);
> +            } else {
> +                /* Write into file from RAM */
> +                assert((sm->flags[i] & VHOST_USER_FS_FLAG_MAP_W));

The vhost-user device backend must not be able to crash the VMM. Please
use an if statement and fail the request if the flags are invalid
instead of assert().

> +                transferred = pwrite(fd, hostptr, mrs_size, 
> sm->fd_offset[i]);
> +            }
> +            trace_vhost_user_fs_slave_io_loop_res(transferred);
> +            if (transferred < 0) {
> +                res = -errno;
> +                break;
> +            }
> +            if (!transferred) {
> +                /* EOF */
> +                break;
> +            }
> +
> +            done += transferred;
> +            len -= transferred;

Is gpa += transferred missing so that this loop can handle crossing
MemoryRegion boundaries?

sm->fd_offset[i] also needs to be put into a local variable and
incremented by transferred each time around the loop.

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