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

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

On Sat, 22 Jun 2019 01:08:40 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 6/21/2019 8:46 PM, Alex Williamson wrote:
> > 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.  

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,

Alex

> > I think resume is going to be
> > directed by the user, writing in the same order they received the
> > data.  Thanks,