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

[Kirti Wankhede]

On 6/21/2019 12:55 AM, Alex Williamson wrote:
> On Thu, 20 Jun 2019 20:07:36 +0530
> Kirti Wankhede <address@hidden> 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 <address@hidden>
>> Reviewed-by: Neo Jia <address@hidden>
>> ---
>>  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.

>  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.

> 
>> +                    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,
>>  };
>>  
>>  /* ---------------------------------------------------------------------- */
>