Re: [PATCH V2 0/7] vdpa live update

[Steven Sistare <steven.sistare@oracle.com>]

On 7/14/2024 10:14 PM, Jason Wang wrote:
> On Fri, Jul 12, 2024 at 9:19 PM Steve Sistare <steven.sistare@oracle.com> wrote:
>>
>> Live update is a technique wherein an application saves its state, exec's
>> to an updated version of itself, and restores its state.  Clients of the
>> application experience a brief suspension of service, on the order of
>> 100's of milliseconds, but are otherwise unaffected.
>>
>> Define and implement interfaces that allow vdpa devices to be preserved
>> across fork or exec, to support live update for applications such as QEMU.
>> The device must be suspended during the update, but its DMA mappings are
>> preserved, so the suspension is brief.
>>
>> The VHOST_NEW_OWNER ioctl transfers device ownership and pinned memory
>> accounting from one process to another.
>>
>> The VHOST_BACKEND_F_NEW_OWNER backend capability indicates that
>> VHOST_NEW_OWNER is supported.
>>
>> The VHOST_IOTLB_REMAP message type updates a DMA mapping with its userland
>> address in the new process.
>>
>> The VHOST_BACKEND_F_IOTLB_REMAP backend capability indicates that
>> VHOST_IOTLB_REMAP is supported and required.  Some devices do not
>> require it, because the userland address of each DMA mapping is discarded
>> after being translated to a physical address.
>>
>> Here is a pseudo-code sequence for performing live update, based on
>> suspend + reset because resume is not yet widely available.  The vdpa device
>> descriptor, fd, remains open across the exec.
>>
>>    ioctl(fd, VHOST_VDPA_SUSPEND)
>>    ioctl(fd, VHOST_VDPA_SET_STATUS, 0)
> 
> I don't understand why we need a reset after suspend, it looks to me
> the previous suspend became meaningless.

The suspend guarantees completion of in-progress DMA.  At least, that is
my interpretation of why that is done for live migration in QEMU, which
also does suspend + reset + re-create.  I am following the live migration
model.

>>    exec
>>
>>    ioctl(fd, VHOST_NEW_OWNER)
>>
>>    issue ioctls to re-create vrings
>>
>>    if VHOST_BACKEND_F_IOTLB_REMAP
> 
> So the idea is for a device that is using a virtual address, it
> doesn't need VHOST_BACKEND_F_IOTLB_REMAP at all?

Actually the reverse: if the device translates virtual to physical when
the mappings are created, and discards the virtual, then VHOST_IOTLB_REMAP
is not needed.

>>        foreach dma mapping
>>            write(fd, {VHOST_IOTLB_REMAP, new_addr})
>>
>>    ioctl(fd, VHOST_VDPA_SET_STATUS,
>>              ACKNOWLEDGE | DRIVER | FEATURES_OK | DRIVER_OK)
> 
>  From API level, this seems to be asymmetric as we have suspending but
> not resuming?

Again, I am just following the path taken by live migration.
I will be happy to use resume when the devices and QEMU support it.
The decision to use reset vs resume should not affect the definition
and use of VHOST_NEW_OWNER and VHOST_IOTLB_REMAP.

- Steve

>> This is faster than VHOST_RESET_OWNER + VHOST_SET_OWNER + VHOST_IOTLB_UPDATE,
>> as that would would unpin and repin physical pages, which would cost multiple
>> seconds for large memories.
>>
>> This is implemented in QEMU by the patch series "Live update: vdpa"
>>    https://lore.kernel.org/qemu-devel/TBD  (reference to be posted shortly)
>>
>> The QEMU implementation leverages the live migration code path, but after
>> CPR exec's new QEMU:
>>    - vhost_vdpa_set_owner() calls VHOST_NEW_OWNER instead of VHOST_SET_OWNER
>>    - vhost_vdpa_dma_map() sets type VHOST_IOTLB_REMAP instead of
>>      VHOST_IOTLB_UPDATE
>>
>> Changes in V2:
>>    - clean up handling of set_map vs dma_map vs platform iommu in remap
>>    - augment and clarify commit messages and comments
>>
>> Steve Sistare (7):
>>    vhost-vdpa: count pinned memory
>>    vhost-vdpa: pass mm to bind
>>    vhost-vdpa: VHOST_NEW_OWNER
>>    vhost-vdpa: VHOST_BACKEND_F_NEW_OWNER
>>    vhost-vdpa: VHOST_IOTLB_REMAP
>>    vhost-vdpa: VHOST_BACKEND_F_IOTLB_REMAP
>>    vdpa/mlx5: new owner capability
>>
>>   drivers/vdpa/mlx5/net/mlx5_vnet.c |   3 +-
>>   drivers/vhost/vdpa.c              | 125 ++++++++++++++++++++++++++++--
>>   drivers/vhost/vhost.c             |  15 ++++
>>   drivers/vhost/vhost.h             |   1 +
>>   include/uapi/linux/vhost.h        |  10 +++
>>   include/uapi/linux/vhost_types.h  |  15 +++-
>>   6 files changed, 161 insertions(+), 8 deletions(-)
>>
>> --
>> 2.39.3
>>
> 
> Thanks
>