> -----Original Message-----
> From: Stefan Hajnoczi <[email protected]>
> Sent: 28 January 2022 08:29
> To: Jag Raman <[email protected]>
> Cc: John Levon <[email protected]>; Thanos Makatos
> <[email protected]>; qemu-devel <[email protected]>;
> Marc-André Lureau <[email protected]>; Philippe Mathieu-Daudé
> <[email protected]>; Paolo Bonzini <[email protected]>; Beraldo Leal
> <[email protected]>; Daniel P. Berrangé <[email protected]>;
> [email protected]; Michael S. Tsirkin <[email protected]>; Marcel
> Apfelbaum <[email protected]>; Eric Blake <[email protected]>;
> Markus Armbruster <[email protected]>; Juan Quintela
> <[email protected]>; Dr . David Alan Gilbert <[email protected]>; Elena
> Ufimtseva <[email protected]>; John Johnson
> <[email protected]>
> Subject: Re: [PATCH v5 17/18] vfio-user: register handlers to facilitate
> migration
>
> On Thu, Jan 27, 2022 at 05:04:26PM +0000, Jag Raman wrote:
> >
> >
> > > On Jan 25, 2022, at 10:48 AM, Stefan Hajnoczi <[email protected]>
> wrote:
> > >
> > > On Wed, Jan 19, 2022 at 04:42:06PM -0500, Jagannathan Raman wrote:
> > >> + * The client subsequetly asks the remote server for any data that
> > >
> > > subsequently
> > >
> > >> +static void vfu_mig_state_running(vfu_ctx_t *vfu_ctx)
> > >> +{
> > >> + VfuObject *o = vfu_get_private(vfu_ctx);
> > >> + VfuObjectClass *k = VFU_OBJECT_GET_CLASS(OBJECT(o));
> > >> + static int migrated_devs;
> > >> + Error *local_err = NULL;
> > >> + int ret;
> > >> +
> > >> + /**
> > >> + * TODO: move to VFU_MIGR_STATE_RESUME handler. Presently, the
> > >> + * VMSD data from source is not available at RESUME state.
> > >> + * Working on a fix for this.
> > >> + */
> > >> + if (!o->vfu_mig_file) {
> > >> + o->vfu_mig_file = qemu_fopen_ops(o, &vfu_mig_fops_load, false);
> > >> + }
> > >> +
> > >> + ret = qemu_remote_loadvm(o->vfu_mig_file);
> > >> + if (ret) {
> > >> + VFU_OBJECT_ERROR(o, "vfu: failed to restore device state");
> > >> + return;
> > >> + }
> > >> +
> > >> + qemu_file_shutdown(o->vfu_mig_file);
> > >> + o->vfu_mig_file = NULL;
> > >> +
> > >> + /* VFU_MIGR_STATE_RUNNING begins here */
> > >> + if (++migrated_devs == k->nr_devs) {
> > >
> > > When is this counter reset so migration can be tried again if it
> > > fails/cancels?
> >
> > Detecting cancellation is a pending item. We will address it in the
> > next rev. Will check with you if we get stuck during the process
> > of implementing it.
> >
> > >
> > >> +static ssize_t vfu_mig_read_data(vfu_ctx_t *vfu_ctx, void *buf,
> > >> + uint64_t size, uint64_t offset)
> > >> +{
> > >> + VfuObject *o = vfu_get_private(vfu_ctx);
> > >> +
> > >> + if (offset > o->vfu_mig_buf_size) {
> > >> + return -1;
> > >> + }
> > >> +
> > >> + if ((offset + size) > o->vfu_mig_buf_size) {
> > >> + warn_report("vfu: buffer overflow - check pending_bytes");
> > >> + size = o->vfu_mig_buf_size - offset;
> > >> + }
> > >> +
> > >> + memcpy(buf, (o->vfu_mig_buf + offset), size);
> > >> +
> > >> + o->vfu_mig_buf_pending -= size;
> > >
> > > This assumes that the caller increments offset by size each time. If
> > > that assumption is okay, then we can just trust offset and don't need to
> > > do arithmetic on vfu_mig_buf_pending. If that assumption is not correct,
> > > then the code needs to be extended to safely update vfu_mig_buf_pending
> > > when offset jumps around arbitrarily between calls.
> >
> > Going by the definition of vfu_migration_callbacks_t in the library, I
> > assumed
> > that read_data advances the offset by size bytes.
> >
> > Will add a comment a comment to explain that.
libvfio-user does not automatically increment offset by size each time, since
the vfio-user client can re-read the migration data multiple times. In
libvfio-user API we state:
Function that is called to read migration data. offset and size can be
any subrange on the offset and size previously returned by prepare_data.
Reading the pending_bytes register is what marks the end of the iteration, and
this is where you need to decrement vfu_mig_buf_pending.
I'll add more unit tests to libvfio-user to validate this behavior.
> >
> > >
> > >> +uint64_t vmstate_vmsd_size(PCIDevice *pci_dev)
> > >> +{
> > >> + DeviceClass *dc = DEVICE_GET_CLASS(DEVICE(pci_dev));
> > >> + const VMStateField *field = NULL;
> > >> + uint64_t size = 0;
> > >> +
> > >> + if (!dc->vmsd) {
> > >> + return 0;
> > >> + }
> > >> +
> > >> + field = dc->vmsd->fields;
> > >> + while (field && field->name) {
> > >> + size += vmstate_size(pci_dev, field);
> > >> + field++;
> > >> + }
> > >> +
> > >> + return size;
> > >> +}
> > >
> > > This function looks incorrect because it ignores subsections as well as
> > > runtime behavior during save(). Although VMStateDescription is partially
> > > declarative, there is still a bunch of imperative code that can write to
> > > the QEMUFile at save() time so there's no way of knowing the size ahead
> > > of time.
> >
> > I see your point, it would be a problem for any field which has the
> > (VMS_BUFFER | VMS_ALLOC) flags set.
> >
> > >
> > > I asked this in a previous revision of this series but I'm not sure if
> > > it was answered: is it really necessary to know the size of the vmstate?
> > > I thought the VFIO migration interface is designed to support
> > > streaming reads/writes. We could choose a fixed size like 64KB and
> > > stream the vmstate in 64KB chunks.
> >
> > The library exposes the migration data to the client as a device BAR with
> > fixed size - the size of which is fixed at boot time, even when using
> > vfu_migration_callbacks_t callbacks.
> >
> > I don’t believe the library supports streaming vmstate/migration-data - see
> > the following comment in migration_region_access() defined in the library:
> >
> > * Does this mean that partial reads are not allowed?
> >
> > Thanos or John,
> >
> > Could you please clarify this?
libvfio-user does support streaming of migration data, this comment is based on
the VFIO documentation:
d. Read data_size bytes of data from (region + data_offset) from the
migration region.
It's not clear to me whether streaming should be allowed, I'd be surprised if
it didn't.
> >
> > Stefan,
> > We attempted to answer the migration cancellation and vmstate size
> > questions previously also, in the following email:
> >
> > https://lore.kernel.org/all/F48606B1-15A4-4DD2-9D71-
> [email protected]/
>
> > libvfio-user has the vfu_migration_callbacks_t interface that allows the
> > device to save/load more data regardless of the size of the migration
> > region. I don't see the issue here since the region doesn't need to be
> > sized to fit the savevm data?
>
> The answer didn't make sense to me:
>
> "In both scenarios at the server end - whether using the migration BAR or
> using callbacks, the migration data is transported to the other end using
> the BAR. As such we need to specify the BAR’s size during initialization.
>
> In the case of the callbacks, the library translates the BAR access to
> callbacks."
>
> The BAR and the migration region within it need a size but my
> understanding is that VFIO migration is designed to stream the device
> state, allowing it to be broken up into multiple reads/writes with
> knowing the device state's size upfront. Here is the description from
> <linux/vfio.h>:
>
> * The sequence to be followed while in pre-copy state and stop-and-copy
> state
> * is as follows:
> * a. Read pending_bytes, indicating the start of a new iteration to get
> device
> * data. Repeated read on pending_bytes at this stage should have no side
> * effects.
> * If pending_bytes == 0, the user application should not iterate to get
> data
> * for that device.
> * If pending_bytes > 0, perform the following steps.
> * b. Read data_offset, indicating that the vendor driver should make data
> * available through the data section. The vendor driver should return
> this
> * read operation only after data is available from (region + data_offset)
> * to (region + data_offset + data_size).
> * c. Read data_size, which is the amount of data in bytes available through
> * the migration region.
> * Read on data_offset and data_size should return the offset and size of
> * the current buffer if the user application reads data_offset and
> * data_size more than once here.
> * d. Read data_size bytes of data from (region + data_offset) from the
> * migration region.
> * e. Process the data.
> * f. Read pending_bytes, which indicates that the data from the previous
> * iteration has been read. If pending_bytes > 0, go to step b.
> *
> * The user application can transition from the _SAVING|_RUNNING
> * (pre-copy state) to the _SAVING (stop-and-copy) state regardless of the
> * number of pending bytes. The user application should iterate in _SAVING
> * (stop-and-copy) until pending_bytes is 0.
>
> This means you can report pending_bytes > 0 until the entire vmstate has
> been read and can pick a fixed chunk size like 64KB for the migration
> region. There's no need to size the migration region to fit the entire
> vmstate.
>
> Stefan
