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

[Kirti Wankhede]

<snip>
>>>>>>>>>> +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.      
>>>>>
>>>>> A)
>>>>>     
>>>>>>> s/resume/save/?    
>>>>>
>>>>> B)
>>>>>      
>>>>>>> Or is this saying that on resume that the vendor driver is requesting a
>>>>>>> specific block of data via data_offset?       
>>>>>>
>>>>>> Correct.    
>>>>>
>>>>> Which one is correct?  Thanks,
>>>>>     
>>>>
>>>> B is correct.  
>>>
>>> Shouldn't data_offset be stored in the migration stream then so we can
>>> at least verify that source and target are in sync?   
>>
>> Why? data_offset is offset within migration region, nothing to do with
>> data stream. While resuming vendor driver can ask data at different
>> offset in migration region.
> 
> So the data is opaque and the sequencing is opaque, the user should
> have no expectation that there's any relationship between where the
> data was read from while saving versus where the target device is
> requesting the next block be written while resuming.  We have a data
> blob and a size and we do what we're told.
> 

That's correct.

>>> I'm not getting a
>>> sense that this protocol involves any sort of sanity or integrity
>>> testing on the vendor driver end, the user can just feed garbage into
>>> the device on resume and watch the results :-\  Thanks,
>>>  
>>
>> vendor driver should be able to do sanity and integrity check within its
>> opaque data. If that sanity fails, return failure for access on field in
>> migration region structure.
> 
> Would that be a synchronous failure on the write of data_size, which
> should result in the device_state moving to invalid?  Thanks,
> 

If data section of migration region is mapped, then on write to
data_size, vendor driver should read staging buffer, validate data and
return sizeof(data_size) if success or return error (< 0). If data
section is trapped, then write on data section should return accordingly
on receiving data. On error, migration/restore would fail.

Thanks,
Kirti