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

[Kirti Wankhede]

On 6/21/2019 8:46 PM, Alex Williamson wrote:
> On Fri, 21 Jun 2019 12:08:26 +0530
> Kirti Wankhede <address@hidden> wrote:
> 
>> 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.
> 
> 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? 

Correct.

Thanks,
Kirti

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