Re: [PATCH v1] docs/devel: Add VFIO device migration documentation

[Alex Williamson]

On Thu, 29 Oct 2020 23:11:16 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> Thanks for corrections Cornelia. I had done the corrections you 
> suggested I had not replied, see my comments on couple of places where I 
> disagree.
> 
> 
> On 10/29/2020 5:22 PM, Cornelia Huck wrote:
> > On Thu, 29 Oct 2020 11:23:11 +0530
> > Kirti Wankhede <kwankhede@nvidia.com> wrote:
> >   
> >> Document interfaces used for VFIO device migration. Added flow of state
> >> changes during live migration with VFIO device.
> >>
> >> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> >> ---
> >>   MAINTAINERS                   |   1 +
> >>   docs/devel/vfio-migration.rst | 119 
> >> ++++++++++++++++++++++++++++++++++++++++++  
> > 
> > You probably want to include this into the Developer's Guide via
> > index.rst.
> >   
> 
> Ok.
> 
> >>   2 files changed, 120 insertions(+)
> >>   create mode 100644 docs/devel/vfio-migration.rst
> >>
> >> diff --git a/MAINTAINERS b/MAINTAINERS
> >> index 6a197bd358d6..6f3fcffc6b3d 100644
> >> --- a/MAINTAINERS
> >> +++ b/MAINTAINERS
> >> @@ -1728,6 +1728,7 @@ M: Alex Williamson <alex.williamson@redhat.com>
> >>   S: Supported
> >>   F: hw/vfio/*
> >>   F: include/hw/vfio/
> >> +F: docs/devel/vfio-migration.rst
> >>   
> >>   vfio-ccw
> >>   M: Cornelia Huck <cohuck@redhat.com>
> >> diff --git a/docs/devel/vfio-migration.rst b/docs/devel/vfio-migration.rst
> >> new file mode 100644
> >> index 000000000000..dab9127825e4
> >> --- /dev/null
> >> +++ b/docs/devel/vfio-migration.rst
> >> @@ -0,0 +1,119 @@
> >> +=====================
> >> +VFIO device Migration
> >> +=====================
> >> +
> >> +VFIO devices use iterative approach for migration because certain VFIO 
> >> devices  
> > 
> > s/use/use an/ ?
> >   
> >> +(e.g. GPU) have large amount of data to be transfered. The iterative 
> >> pre-copy
> >> +phase of migration allows for the guest to continue whilst the VFIO 
> >> device state
> >> +is transferred to destination, this helps to reduce the total downtime of 
> >> the  
> > 
> > s/to destination,/to the destination;/
> >   
> >> +VM. VFIO devices can choose to skip the pre-copy phase of migration by 
> >> returning
> >> +pending_bytes as zero during pre-copy phase.  
> > 
> > s/during/during the/
> >   
> >> +
> >> +Detailed description of UAPI for VFIO device for migration is in the 
> >> comment
> >> +above ``vfio_device_migration_info`` structure definition in header file
> >> +linux-headers/linux/vfio.h.  
> > 
> > I think I'd copy that to this file. If I'm looking at the
> > documentation, I'd rather not go hunting for source code to find out
> > what structure you are talking about. Plus, as it's UAPI, I don't
> > expect it to change much, so it should be easy to keep the definitions
> > in sync (famous last words).
> >   
> 
> I feel its duplication of documentation. I would like to know others 
> views as well.


TBH I don't think it's necessary here either, we're documenting the
QEMU interaction with the uAPI, the uAPI itself is documented in the
kernel header.  I don't think it would be unreasonable to ask someone
trying to understand this to look at both sources together.


> >> +
> >> +VFIO device hooks for iterative approach:
> >> +-  A ``save_setup`` function that setup migration region, sets _SAVING 
> >> flag in  
> > 
> > s/setup/sets up the/
> > s/in/in the/
> >   
> >> +VFIO device state and inform VFIO IOMMU module to start dirty page 
> >> tracking.  
> > 
> > s/inform/informs the/
> >   
> >> +
> >> +- A ``load_setup`` function that setup migration region on the 
> >> destination and  
> > 
> > s/setup/sets up the/
> >   
> >> +sets _RESUMING flag in VFIO device state.  
> > 
> > s/in/in the/
> >   
> >> +
> >> +- A ``save_live_pending`` function that reads pending_bytes from vendor 
> >> driver
> >> +that indicate how much more data the vendor driver yet to save for the 
> >> VFIO
> >> +device.  
> > 
> > "A ``save_live_pending`` function that reads pending_bytes from the
> > vendor driver, which indicates the amount of data that the vendor
> > driver has yet to save for the VFIO device." ?
> >   
> >> +
> >> +- A ``save_live_iterate`` function that reads VFIO device's data from 
> >> vendor  
> > 
> > s/reads/reads the/
> > s/from/from the/
> >   
> >> +driver through migration region during iterative phase.  
> > 
> > s/through/through the/
> >   
> >> +
> >> +- A ``save_live_complete_precopy`` function that resets _RUNNING flag 
> >> from VFIO  
> > 
> > s/from/from the/
> >   
> >> +device state, saves device config space, if any, and iteratively copies  
> > 
> > s/saves/saves the/
> >   
> >> +remaining data for VFIO device till pending_bytes returned by vendor 
> >> driver
> >> +is zero.  
> > 
> > "...and interactively copies the remaining data for the VFIO device
> > until the vendor driver indicates that no data remains (pending_bytes
> > is zero)." ?


Connie, was that intentional to replace "iteratively" with
"interactively"?  Iteratively seems correct to me.


> >   
> >> +
> >> +- A ``load_state`` function loads config section and data sections 
> >> generated by
> >> +above save functions.  
> > 
> > "A ``load_state`` function that loads the config section and the data
> > sections that are generated by the save functions above." ?
> >   
> >> +
> >> +- ``cleanup`` functions for both save and load that unmap migration 
> >> region.  
> > 
> > ..."that perform any migration-related cleanup, including unmapping the
> > migration region." ?
> >   
> >> +
> >> +VM state change handler is registered to change VFIO device state based 
> >> on VM
> >> +state change.  
> > 
> > "A VM state change handler is registered to change the VFIO device
> > state when the VM state changes." ?
> >   
> >> +
> >> +Similarly, a migration state change notifier is added to get a 
> >> notification on  
> > 
> > s/added/registered/ ?
> >   
> >> +migration state change. These states are translated to VFIO device state 
> >> and
> >> +conveyed to vendor driver.
> >> +
> >> +System memory dirty pages tracking
> >> +----------------------------------
> >> +
> >> +A ``log_sync`` memory listener callback is added to mark system memory 
> >> pages  
> > 
> > s/is added to mark/marks those/
> >   
> >> +as dirty which are used for DMA by VFIO device. Dirty pages bitmap is 
> >> queried  
> > 
> > s/by/by the/
> > s/Dirty/The dirty/
> >   
> >> +per container. All pages pinned by vendor driver through vfio_pin_pages() 
> >>  
> > 
> > s/by/by the/
> >   
> >> +external API have to be marked as dirty during migration. When there are 
> >> CPU
> >> +writes, CPU dirty page tracking can identify dirtied pages, but any page 
> >> pinned
> >> +by vendor driver can also be written by device. There is currently no 
> >> device  
> > 
> > s/by/by the/ (x2)
> >   
> >> +which has hardware support for dirty page tracking. So all pages which are
> >> +pinned by vendor driver are considered as dirty.
> >> +Dirty pages are tracked when device is in stop-and-copy phase because if 
> >> pages
> >> +are marked dirty during pre-copy phase and content is transfered from 
> >> source to
> >> +destination, there is no way to know newly dirtied pages from the point 
> >> they
> >> +were copied earlier until device stops. To avoid repeated copy of same 
> >> content,
> >> +pinned pages are marked dirty only during stop-and-copy phase.  
>
>
> > Let me take a quick stab at rewriting this paragraph (not sure if I
> > understood it correctly):
> > 
> > "Dirty pages are tracked when the device is in the stop-and-copy phase.
> > During the pre-copy phase, it is not possible to distinguish a dirty
> > page that has been transferred from the source to the destination from
> > newly dirtied pages, which would lead to repeated copying of the same
> > content. Therefore, pinned pages are only marked dirty during the
> > stop-and-copy phase." ?
> >   
> 
> I think above rephrase only talks about repeated copying in pre-copy 
> phase. Used "copied earlier until device stops" to indicate both 
> pre-copy and stop-and-copy till device stops.


Now I'm confused, I thought we had abandoned the idea that we can only
report pinned pages during stop-and-copy.  Doesn't the device needs to
expose its dirty memory footprint during the iterative phase regardless
of whether that causes repeat copies?  If QEMU iterates and sees that
all memory is still dirty, it may have transferred more data, but it
can actually predict if it can achieve its downtime tolerances.  Which
is more important, less data transfer or predictability?  Thanks,

Alex


> >> +
> >> +System memory dirty pages tracking when vIOMMU is enabled
> >> +---------------------------------------------------------
> >> +With vIOMMU, IO virtual address range can get unmapped while in pre-copy 
> >> phase  
> > 
> > s/IO/an I/O/
> >   
> >> +of migration. In that case, unmap ioctl returns pages pinned in that 
> >> range and  
> > 
> > s/unmap ioctl returns pages pinned/the unmap ioctl returns any pinned 
> > pages/ ?
> >   
> >> +QEMU reports corresponding guest physical pages dirty.
> >> +During stop-and-copy phase, an IOMMU notifier is used to get a callback 
> >> for
> >> +mapped pages and then dirty pages bitmap is fetched from VFIO IOMMU 
> >> modules for
> >> +those mapped ranges.
> >> +
> >> +Flow of state changes during Live migration
> >> +===========================================
> >> +Below is the flow of state change during live migration where states in 
> >> brackets
> >> +represent VM state, migration state and VFIO device state as:
> >> +                (VM state, MIGRATION_STATUS, VFIO_DEVICE_STATE)  
> > 
> > "The values in the brackets represent the VM state, the migration
> > state, and the VFIO device state, respectively." ?
> >   
> >> +
> >> +Live migration save path
> >> +------------------------
> >> +                        QEMU normal running state
> >> +                        (RUNNING, _NONE, _RUNNING)
> >> +                                    |
> >> +                       migrate_init spawns migration_thread
> >> +                Migration thread then calls each device's .save_setup()
> >> +                        (RUNNING, _SETUP, _RUNNING|_SAVING)
> >> +                                    |
> >> +                        (RUNNING, _ACTIVE, _RUNNING|_SAVING)
> >> +            If device is active, get pending_bytes by .save_live_pending()
> >> +         if total pending_bytes >= threshold_size, call 
> >> .save_live_iterate()
> >> +                  Data of VFIO device for pre-copy phase is copied
> >> +     Iterate till total pending bytes converge and are less than threshold
> >> +                                    |
> >> +   On migration completion, vCPUs stops and calls 
> >> .save_live_complete_precopy  
> > 
> > s/vCPUs/vCPU/ ?
> >   
> >> +   for each active device. VFIO device is then transitioned in _SAVING 
> >> state  
> > 
> > s/VFIO device/The VFIO device/
> > 
> > s/in/into/
> >   
> >> +                    (FINISH_MIGRATE, _DEVICE, _SAVING)
> >> +                                    |
> >> +For VFIO device, iterate in .save_live_complete_precopy until pending 
> >> data is 0  
> > 
> > s/For/For the/
> >   
> >> +                    (FINISH_MIGRATE, _DEVICE, _STOPPED)
> >> +                                    |
> >> +                    (FINISH_MIGRATE, _COMPLETED, _STOPPED)
> >> +                Migraton thread schedule cleanup bottom half and exit  
> > 
> > s/schedule/schedules/
> > s/exit/exits/
> >   
> >> +
> >> +Live migration resume path
> >> +--------------------------
> >> +
> >> +             Incoming migration calls .load_setup for each device
> >> +                        (RESTORE_VM, _ACTIVE, _STOPPED)
> >> +                                    |
> >> +    For each device, .load_state is called for that device section data
> >> +                        (RESTORE_VM, _ACTIVE, _RESUMING)
> >> +                                    |
> >> +    At the end, called .load_cleanup for each device and vCPUs are 
> >> started                        |  
> > 
> > Stray '|', probably should go on the next line?
> > 
> > s/called .load_cleanup/.load_cleanup is called/
> > 
> >   
> >> +                        (RUNNING, _NONE, _RUNNING)
> >> +
> >> +
> >> +Postcopy
> >> +========
> >> +Postcopy migration is not supported for VFIO devices.  
> > 
> > s/not/not yet/, I think? (Disregarding any time frame here.)
> > 
> > Generally, I think this is useful information to have in the docs.
> >   
>