Re: [Qemu-devel] [PATCH v4 08/13] vfio: Add save state functions to SaveVMHandlers

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 21 Jun 2019 12:08:26 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 6/21/2019 12:55 AM, Alex Williamson wrote:
> > On Thu, 20 Jun 2019 20:07:36 +0530
> > Kirti Wankhede <kwankhede@nvidia.com> wrote:
> >   
> >> Added .save_live_pending, .save_live_iterate and .save_live_complete_precopy
> >> functions. These functions handles pre-copy and stop-and-copy phase.
> >>
> >> In _SAVING|_RUNNING device state or pre-copy phase:
> >> - read pending_bytes
> >> - read data_offset - indicates kernel driver to write data to staging
> >>   buffer which is mmapped.  
> > 
> > Why is data_offset the trigger rather than data_size?  It seems that
> > data_offset can't really change dynamically since it might be mmap'd,
> > so it seems unnatural to bother re-reading it.
> >   
> 
> Vendor driver can change data_offset, he can have different data_offset
> for device data and dirty pages bitmap.
> 
> >> - read data_size - amount of data in bytes written by vendor driver in migration
> >>   region.
> >> - if data section is trapped, pread() number of bytes in data_size, from
> >>   data_offset.
> >> - if data section is mmaped, read mmaped buffer of size data_size.
> >> - Write data packet to file stream as below:
> >> {VFIO_MIG_FLAG_DEV_DATA_STATE, data_size, actual data,
> >> VFIO_MIG_FLAG_END_OF_STATE }
> >>
> >> In _SAVING device state or stop-and-copy phase
> >> a. read config space of device and save to migration file stream. This
> >>    doesn't need to be from vendor driver. Any other special config state
> >>    from driver can be saved as data in following iteration.
> >> b. read pending_bytes - indicates kernel driver to write data to staging
> >>    buffer which is mmapped.  
> > 
> > Is it pending_bytes or data_offset that triggers the write out of
> > data?  Why pending_bytes vs data_size?  I was interpreting
> > pending_bytes as the total data size while data_size is the size
> > available to read now, so assumed data_size would be more closely
> > aligned to making the data available.
> >   
> 
> Sorry, that's my mistake while editing, its read data_offset as in above
> case.
> 
> >> c. read data_size - amount of data in bytes written by vendor driver in
> >>    migration region.
> >> d. if data section is trapped, pread() from data_offset of size data_size.
> >> e. if data section is mmaped, read mmaped buffer of size data_size.  
> > 
> > Should this read as "pread() from data_offset of data_size, or
> > optionally if mmap is supported on the data area, read data_size from
> > start of mapped buffer"?  IOW, pread should always work.  Same in
> > previous section.
> >   
> 
> ok. I'll update.
> 
> >> f. Write data packet as below:
> >>    {VFIO_MIG_FLAG_DEV_DATA_STATE, data_size, actual data}
> >> g. iterate through steps b to f until (pending_bytes > 0)  
> > 
> > s/until/while/  
> 
> Ok.
> 
> >   
> >> h. Write {VFIO_MIG_FLAG_END_OF_STATE}
> >>
> >> .save_live_iterate runs outside the iothread lock in the migration case, which
> >> could race with asynchronous call to get dirty page list causing data corruption
> >> in mapped migration region. Mutex added here to serial migration buffer read
> >> operation.  
> > 
> > Would we be ahead to use different offsets within the region for device
> > data vs dirty bitmap to avoid this?
> >  
> 
> Lock will still be required to serialize the read/write operations on
> vfio_device_migration_info structure in the region.
> 
> 
> >> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> >> Reviewed-by: Neo Jia <cjia@nvidia.com>
> >> ---
> >>  hw/vfio/migration.c | 212 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >>  1 file changed, 212 insertions(+)
> >>
> >> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> >> index fe0887c27664..0a2f30872316 100644
> >> --- a/hw/vfio/migration.c
> >> +++ b/hw/vfio/migration.c
> >> @@ -107,6 +107,111 @@ static int vfio_migration_set_state(VFIODevice *vbasedev, uint32_t state)
> >>      return 0;
> >>  }
> >>  
> >> +static int vfio_save_buffer(QEMUFile *f, VFIODevice *vbasedev)
> >> +{
> >> +    VFIOMigration *migration = vbasedev->migration;
> >> +    VFIORegion *region = &migration->region.buffer;
> >> +    uint64_t data_offset = 0, data_size = 0;
> >> +    int ret;
> >> +
> >> +    ret = pread(vbasedev->fd, &data_offset, sizeof(data_offset),
> >> +                region->fd_offset + offsetof(struct vfio_device_migration_info,
> >> +                                             data_offset));
> >> +    if (ret != sizeof(data_offset)) {
> >> +        error_report("Failed to get migration buffer data offset %d",
> >> +                     ret);
> >> +        return -EINVAL;
> >> +    }
> >> +
> >> +    ret = pread(vbasedev->fd, &data_size, sizeof(data_size),
> >> +                region->fd_offset + offsetof(struct vfio_device_migration_info,
> >> +                                             data_size));
> >> +    if (ret != sizeof(data_size)) {
> >> +        error_report("Failed to get migration buffer data size %d",
> >> +                     ret);
> >> +        return -EINVAL;
> >> +    }
> >> +
> >> +    if (data_size > 0) {
> >> +        void *buf = NULL;
> >> +        bool buffer_mmaped = false;
> >> +
> >> +        if (region->mmaps) {
> >> +            int i;
> >> +
> >> +            for (i = 0; i < region->nr_mmaps; i++) {
> >> +                if ((data_offset >= region->mmaps[i].offset) &&
> >> +                    (data_offset < region->mmaps[i].offset +
> >> +                                   region->mmaps[i].size)) {
> >> +                    buf = region->mmaps[i].mmap + (data_offset -
> >> +                                                   region->mmaps[i].offset);  
> > 
> > So you're expecting that data_offset is somewhere within the data
> > area.  Why doesn't the data always simply start at the beginning of the
> > data area?  ie. data_offset would coincide with the beginning of the
> > mmap'able area (if supported) and be static.  Does this enable some
> > functionality in the vendor driver?  
> 
> Do you want to enforce that to vendor driver?
> From the feedback on previous version I thought vendor driver should
> define data_offset within the region
> "I'd suggest that the vendor driver expose a read-only
> data_offset that matches a sparse mmap capability entry should the
> driver support mmap.  The use should always read or write data from the
> vendor defined data_offset"
> 
> This also adds flexibility to vendor driver such that vendor driver can
> define different data_offset for device data and dirty page bitmap
> within same mmaped region.

I agree, it adds flexibility, the protocol was not evident to me until
I got here though.

> >  Does resume data need to be
> > written from the same offset where it's read?  
> 
> No, resume data should be written from the data_offset that vendor
> driver provided during resume.

s/resume/save/?

Or is this saying that on resume that the vendor driver is requesting a
specific block of data via data_offset?  I think resume is going to be
directed by the user, writing in the same order they received the
data.  Thanks,

Alex

> >> +                    buffer_mmaped = true;
> >> +                    break;
> >> +                }
> >> +            }
> >> +        }
> >> +
> >> +        if (!buffer_mmaped) {
> >> +            buf = g_malloc0(data_size);
> >> +            ret = pread(vbasedev->fd, buf, data_size,
> >> +                        region->fd_offset + data_offset);
> >> +            if (ret != data_size) {
> >> +                error_report("Failed to get migration data %d", ret);
> >> +                g_free(buf);
> >> +                return -EINVAL;
> >> +            }
> >> +        }
> >> +
> >> +        qemu_put_be64(f, data_size);
> >> +        qemu_put_buffer(f, buf, data_size);
> >> +
> >> +        if (!buffer_mmaped) {
> >> +            g_free(buf);
> >> +        }
> >> +        migration->pending_bytes -= data_size;
> >> +    } else {
> >> +        qemu_put_be64(f, data_size);
> >> +    }
> >> +
> >> +    ret = qemu_file_get_error(f);
> >> +
> >> +    return data_size;
> >> +}
> >> +
> >> +static int vfio_update_pending(VFIODevice *vbasedev)
> >> +{
> >> +    VFIOMigration *migration = vbasedev->migration;
> >> +    VFIORegion *region = &migration->region.buffer;
> >> +    uint64_t pending_bytes = 0;
> >> +    int ret;
> >> +
> >> +    ret = pread(vbasedev->fd, &pending_bytes, sizeof(pending_bytes),
> >> +                region->fd_offset + offsetof(struct vfio_device_migration_info,
> >> +                                             pending_bytes));  
> > 
> > Did this trigger the vendor driver to write out to the data area when
> > we don't need it to?
> >   
> 
> No, as I mentioned above, I'll update the description.
> 
> Thanks,
> Kirti
> 
> >> +    if ((ret < 0) || (ret != sizeof(pending_bytes))) {
> >> +        error_report("Failed to get pending bytes %d", ret);
> >> +        migration->pending_bytes = 0;
> >> +        return (ret < 0) ? ret : -EINVAL;
> >> +    }
> >> +
> >> +    migration->pending_bytes = pending_bytes;
> >> +    return 0;
> >> +}
> >> +
> >> +static int vfio_save_device_config_state(QEMUFile *f, void *opaque)
> >> +{
> >> +    VFIODevice *vbasedev = opaque;
> >> +
> >> +    qemu_put_be64(f, VFIO_MIG_FLAG_DEV_CONFIG_STATE);
> >> +
> >> +    if (vbasedev->type == VFIO_DEVICE_TYPE_PCI) {
> >> +        vfio_pci_save_config(vbasedev, f);
> >> +    }
> >> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
> >> +
> >> +    return qemu_file_get_error(f);
> >> +}
> >> +
> >>  /* ---------------------------------------------------------------------- */
> >>  
> >>  static int vfio_save_setup(QEMUFile *f, void *opaque)
> >> @@ -163,9 +268,116 @@ static void vfio_save_cleanup(void *opaque)
> >>      }
> >>  }
> >>  
> >> +static void vfio_save_pending(QEMUFile *f, void *opaque,
> >> +                              uint64_t threshold_size,
> >> +                              uint64_t *res_precopy_only,
> >> +                              uint64_t *res_compatible,
> >> +                              uint64_t *res_postcopy_only)
> >> +{
> >> +    VFIODevice *vbasedev = opaque;
> >> +    VFIOMigration *migration = vbasedev->migration;
> >> +    int ret;
> >> +
> >> +    ret = vfio_update_pending(vbasedev);
> >> +    if (ret) {
> >> +        return;
> >> +    }
> >> +
> >> +    if (vbasedev->device_state & VFIO_DEVICE_STATE_RUNNING) {
> >> +        *res_precopy_only += migration->pending_bytes;
> >> +    } else {
> >> +        *res_postcopy_only += migration->pending_bytes;
> >> +    }
> >> +    *res_compatible += 0;
> >> +}
> >> +
> >> +static int vfio_save_iterate(QEMUFile *f, void *opaque)
> >> +{
> >> +    VFIODevice *vbasedev = opaque;
> >> +    VFIOMigration *migration = vbasedev->migration;
> >> +    int ret;
> >> +
> >> +    qemu_put_be64(f, VFIO_MIG_FLAG_DEV_DATA_STATE);
> >> +
> >> +    qemu_mutex_lock(&migration->lock);
> >> +    ret = vfio_save_buffer(f, vbasedev);
> >> +    qemu_mutex_unlock(&migration->lock);
> >> +
> >> +    if (ret < 0) {
> >> +        error_report("vfio_save_buffer failed %s",
> >> +                     strerror(errno));
> >> +        return ret;
> >> +    }
> >> +
> >> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
> >> +
> >> +    ret = qemu_file_get_error(f);
> >> +    if (ret) {
> >> +        return ret;
> >> +    }
> >> +
> >> +    return ret;
> >> +}
> >> +
> >> +static int vfio_save_complete_precopy(QEMUFile *f, void *opaque)
> >> +{
> >> +    VFIODevice *vbasedev = opaque;
> >> +    VFIOMigration *migration = vbasedev->migration;
> >> +    int ret;
> >> +
> >> +    ret = vfio_migration_set_state(vbasedev, VFIO_DEVICE_STATE_SAVING);
> >> +    if (ret) {
> >> +        error_report("Failed to set state STOP and SAVING");
> >> +        return ret;
> >> +    }
> >> +
> >> +    ret = vfio_save_device_config_state(f, opaque);
> >> +    if (ret) {
> >> +        return ret;
> >> +    }
> >> +
> >> +    ret = vfio_update_pending(vbasedev);
> >> +    if (ret) {
> >> +        return ret;
> >> +    }
> >> +
> >> +    while (migration->pending_bytes > 0) {
> >> +        qemu_put_be64(f, VFIO_MIG_FLAG_DEV_DATA_STATE);
> >> +        ret = vfio_save_buffer(f, vbasedev);
> >> +        if (ret < 0) {
> >> +            error_report("Failed to save buffer");
> >> +            return ret;
> >> +        } else if (ret == 0) {
> >> +            break;
> >> +        }
> >> +
> >> +        ret = vfio_update_pending(vbasedev);
> >> +        if (ret) {
> >> +            return ret;
> >> +        }
> >> +    }
> >> +
> >> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
> >> +
> >> +    ret = qemu_file_get_error(f);
> >> +    if (ret) {
> >> +        return ret;
> >> +    }
> >> +
> >> +    ret = vfio_migration_set_state(vbasedev, VFIO_DEVICE_STATE_STOPPED);
> >> +    if (ret) {
> >> +        error_report("Failed to set state STOPPED");
> >> +        return ret;
> >> +    }
> >> +    return ret;
> >> +}
> >> +
> >>  static SaveVMHandlers savevm_vfio_handlers = {
> >>      .save_setup = vfio_save_setup,
> >>      .save_cleanup = vfio_save_cleanup,
> >> +    .save_live_pending = vfio_save_pending,
> >> +    .save_live_iterate = vfio_save_iterate,
> >> +    .save_live_complete_precopy = vfio_save_complete_precopy,
> >>  };
> >>  
> >>  /* ---------------------------------------------------------------------- */  
> >