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Subject: Re: [PATCH v9 Kernel 1/5] vfio: KABI for migration interface for
 device state
To:     Yan Zhao <yan.y.zhao@intel.com>
CC:     Alex Williamson <alex.williamson@redhat.com>,
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From:   Kirti Wankhede <kwankhede@nvidia.com>
Message-ID: <5592adf5-7df8-6e1a-b178-87aeee427a6a@nvidia.com>
Date:   Fri, 15 Nov 2019 00:25:42 +0530
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>>>>> + * Vendor driver should decide whether to partition data section and how to
>>>>> + * partition the data section. Vendor driver should return data_offset
>>>>> + * accordingly.
>>>>> + *
>>>>> + * Sequence to be followed for _SAVING|_RUNNING device state or pre-copy phase
>>>>> + * and for _SAVING device state or stop-and-copy phase:
>>>>> + * a. read pending_bytes. If pending_bytes > 0, go through below steps.
>>>>> + * b. read data_offset, indicates kernel driver to write data to staging buffer.
>>>>> + *    Kernel driver should return this read operation only after writing data to
>>>>> + *    staging buffer is done.
>>> May I know under what condition this data_offset will be changed per
>>> each iteration from a-f ?
>>>
>>
>> Its upto vendor driver, if vendor driver maintains multiple partitions
>> in data section.
>>
> So, do you mean each time before doing b (reading data_offset), step a
> (reading pending_bytes) is mandatory, otherwise the vendor driver does
> not know which data_offset is?

pending_bytes will only tell bytes remaining to transfer from vendor 
driver. On read operation on data_offset, vendor driver should decide 
what to send depending on where he is making data available to userspace.

> Then, any lock to wrap step a and b to ensure atomic?

With current QEMU implementation, where migration is single thread, 
there is not need of lock yet. But when we add multi-threaded support 
may be in future then locks will be required in userspace side.

Thanks,
Kirti