Re: [PATCH v16 QEMU 09/16] vfio: Add save state functions to SaveVMHandlers

[Kirti Wankhede <kwankhede@nvidia.com>]

On 5/5/2020 10:07 AM, Alex Williamson wrote:
> On Tue, 5 May 2020 04:48:14 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
>> On 3/26/2020 3:33 AM, Alex Williamson wrote:
>>> On Wed, 25 Mar 2020 02:39:07 +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. If pending_bytes > 0, go through below steps.
>>>> - read data_offset - indicates kernel driver to write data to staging
>>>>     buffer.
>>>> - read data_size - amount of data in bytes written by vendor driver in
>>>>     migration region.
>>>> - read data_size bytes of data from data_offset in the migration region.
>>>> - 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. If pending_bytes > 0, go through below steps.
>>>> c. read data_offset - indicates kernel driver to write data to staging
>>>>      buffer.
>>>> d. read data_size - amount of data in bytes written by vendor driver in
>>>>      migration region.
>>>> e. read data_size bytes of data from data_offset in the migration region.
>>>> f. Write data packet as below:
>>>>      {VFIO_MIG_FLAG_DEV_DATA_STATE, data_size, actual data}
>>>> g. iterate through steps b to f while (pending_bytes > 0)
>>>> h. Write {VFIO_MIG_FLAG_END_OF_STATE}
>>>>
>>>> When data region is mapped, its user's responsibility to read data from
>>>> data_offset of data_size before moving to next steps.
>>>>
>>>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>>>> Reviewed-by: Neo Jia <cjia@nvidia.com>
>>>> ---
>>>>    hw/vfio/migration.c           | 245 +++++++++++++++++++++++++++++++++++++++++-
>>>>    hw/vfio/trace-events          |   6 ++
>>>>    include/hw/vfio/vfio-common.h |   1 +
>>>>    3 files changed, 251 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
>>>> index 033f76526e49..ecbeed5182c2 100644
>>>> --- a/hw/vfio/migration.c
>>>> +++ b/hw/vfio/migration.c
>>>> @@ -138,6 +138,137 @@ static int vfio_migration_set_state(VFIODevice *vbasedev, uint32_t mask,
>>>>        return 0;
>>>>    }
>>>>    
>>>> +static void *find_data_region(VFIORegion *region,
>>>> +                              uint64_t data_offset,
>>>> +                              uint64_t data_size)
>>>> +{
>>>> +    void *ptr = NULL;
>>>> +    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) &&
>>>> +            (data_size <= region->mmaps[i].size)) {
>>>
>>> (data_offset - region->mmaps[i].offset) can be non-zero, so this test
>>> is invalid.  Additionally the uapi does not require that a give data
>>> chunk fits exclusively within an mmap'd area, it may overlap one or
>>> more mmap'd sections of the region, possibly with non-mmap'd areas
>>> included.
>>>    
>>
>> What's the advantage of having mmap and non-mmap overlapped regions?
>> Isn't it better to have data section either mapped or trapped?
> 
> The spec allows for it, therefore we need to support it.  A vendor
> driver might choose to include a header with sequence and checksum
> information for each transaction, they might accomplish this by setting
> data_offset to a trapped area backed by kernel memory followed by an
> area supporting direct mmap to the device.  The target end could then
> fault on writing the header if the sequence information is incorrect.
> A trapped area at the end of the transaction could allow the vendor
> driver to validate a checksum.
> 

If mmap and non-mmap regions overlapped is allowed then here read() 
should be used, which means buffer is allocated, then get data in buffer 
(first memcpy) and then call qemu_put_buffer(f, buf, data_size) (second 
memcpy)

Advantage of using full mmaped region for data, qemu_put_buffer(f, buf, 
data_size) directly uses pointer to mmaped region and so we reduce one 
memcpy.

>>>> +            ptr = region->mmaps[i].mmap + (data_offset -
>>>> +                                           region->mmaps[i].offset);
>>>> +            break;
>>>> +        }
>>>> +    }
>>>> +    return ptr;
>>>> +}
>>>> +
>>>> +static int vfio_save_buffer(QEMUFile *f, VFIODevice *vbasedev)
>>>> +{
>>>> +    VFIOMigration *migration = vbasedev->migration;
>>>> +    VFIORegion *region = &migration->region;
>>>> +    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("%s: Failed to get migration buffer data offset %d",
>>>> +                     vbasedev->name, 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("%s: Failed to get migration buffer data size %d",
>>>> +                     vbasedev->name, ret);
>>>> +        return -EINVAL;
>>>> +    }
>>>> +
>>>> +    if (data_size > 0) {
>>>> +        void *buf = NULL;
>>>> +        bool buffer_mmaped;
>>>> +
>>>> +        if (region->mmaps) {
>>>> +            buf = find_data_region(region, data_offset, data_size);
>>>> +        }
>>>> +
>>>> +        buffer_mmaped = (buf != NULL) ? true : false;
>>>
>>> The ternary is unnecessary, "? true : false" is redundant.
>>>    
>>
>> Removing it.
>>
>>>> +
>>>> +        if (!buffer_mmaped) {
>>>> +            buf = g_try_malloc0(data_size);
>>>
>>> Why do we need zero'd memory?
>>>    
>>
>> Zeroed memory not required, removing 0
>>
>>>> +            if (!buf) {
>>>> +                error_report("%s: Error allocating buffer ", __func__);
>>>> +                return -ENOMEM;
>>>> +            }
>>>> +
>>>> +            ret = pread(vbasedev->fd, buf, data_size,
>>>> +                        region->fd_offset + data_offset);
>>>> +            if (ret != data_size) {
>>>> +                error_report("%s: Failed to get migration data %d",
>>>> +                             vbasedev->name, ret);
>>>> +                g_free(buf);
>>>> +                return -EINVAL;
>>>> +            }
>>>> +        }
>>>> +
>>>> +        qemu_put_be64(f, data_size);
>>>> +        qemu_put_buffer(f, buf, data_size);
>>>
>>> This can segfault when mmap'd given the above assumptions about size
>>> and layout.
>>>    
>>>> +
>>>> +        if (!buffer_mmaped) {
>>>> +            g_free(buf);
>>>> +        }
>>>> +    } else {
>>>> +        qemu_put_be64(f, data_size);
>>>
>>> We insert a zero?  Couldn't we add the section header and end here and
>>> skip it entirely?
>>>    
>>
>> This is used during resuming, data_size 0 indicates end of data.
>>
>>>> +    }
>>>> +
>>>> +    trace_vfio_save_buffer(vbasedev->name, data_offset, data_size,
>>>> +                           migration->pending_bytes);
>>>> +
>>>> +    ret = qemu_file_get_error(f);
>>>> +    if (ret) {
>>>> +        return ret;
>>>> +    }
>>>> +
>>>> +    return data_size;
>>>> +}
>>>> +
>>>> +static int vfio_update_pending(VFIODevice *vbasedev)
>>>> +{
>>>> +    VFIOMigration *migration = vbasedev->migration;
>>>> +    VFIORegion *region = &migration->region;
>>>> +    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));
>>>> +    if ((ret < 0) || (ret != sizeof(pending_bytes))) {
>>>> +        error_report("%s: Failed to get pending bytes %d",
>>>> +                     vbasedev->name, ret);
>>>> +        migration->pending_bytes = 0;
>>>> +        return (ret < 0) ? ret : -EINVAL;
>>>> +    }
>>>> +
>>>> +    migration->pending_bytes = pending_bytes;
>>>> +    trace_vfio_update_pending(vbasedev->name, 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->ops && vbasedev->ops->vfio_save_config) {
>>>> +        vbasedev->ops->vfio_save_config(vbasedev, f);
>>>> +    }
>>>> +
>>>> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
>>>> +
>>>> +    trace_vfio_save_device_config_state(vbasedev->name);
>>>> +
>>>> +    return qemu_file_get_error(f);
>>>> +}
>>>> +
>>>>    /* ---------------------------------------------------------------------- */
>>>>    
>>>>    static int vfio_save_setup(QEMUFile *f, void *opaque)
>>>> @@ -154,7 +285,7 @@ static int vfio_save_setup(QEMUFile *f, void *opaque)
>>>>            qemu_mutex_unlock_iothread();
>>>>            if (ret) {
>>>>                error_report("%s: Failed to mmap VFIO migration region %d: %s",
>>>> -                         vbasedev->name, migration->region.index,
>>>> +                         vbasedev->name, migration->region.nr,
>>>>                             strerror(-ret));
>>>>                return ret;
>>>>            }
>>>> @@ -194,9 +325,121 @@ static void vfio_save_cleanup(void *opaque)
>>>>        trace_vfio_save_cleanup(vbasedev->name);
>>>>    }
>>>>    
>>>> +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;
>>>> +    }
>>>> +
>>>> +    *res_precopy_only += migration->pending_bytes;
>>>> +
>>>> +    trace_vfio_save_pending(vbasedev->name, *res_precopy_only,
>>>> +                            *res_postcopy_only, *res_compatible);
>>>> +}
>>>> +
>>>> +static int vfio_save_iterate(QEMUFile *f, void *opaque)
>>>> +{
>>>> +    VFIODevice *vbasedev = opaque;
>>>> +    int ret, data_size;
>>>> +
>>>> +    qemu_put_be64(f, VFIO_MIG_FLAG_DEV_DATA_STATE);
>>>> +
>>>> +    data_size = vfio_save_buffer(f, vbasedev);
>>>> +
>>>> +    if (data_size < 0) {
>>>> +        error_report("%s: vfio_save_buffer failed %s", vbasedev->name,
>>>> +                     strerror(errno));
>>>> +        return data_size;
>>>> +    }
>>>> +
>>>> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
>>>> +
>>>> +    ret = qemu_file_get_error(f);
>>>> +    if (ret) {
>>>> +        return ret;
>>>> +    }
>>>> +
>>>> +    trace_vfio_save_iterate(vbasedev->name, data_size);
>>>> +    if (data_size == 0) {
>>>> +        /* indicates data finished, goto complete phase */
>>>> +        return 1;
>>>
>>> But it's pending_bytes not data_size that indicates we're done.  How do
>>> we get away with ignoring pending_bytes for the save_live_iterate phase?
>>>    
>>
>> This is requirement mentioned above qemu_savevm_state_iterate() which
>> calls .save_live_iterate.
>>
>> /*	
>>    * this function has three return values:
>>    *   negative: there was one error, and we have -errno.
>>    *   0 : We haven't finished, caller have to go again
>>    *   1 : We have finished, we can go to complete phase
>>    */
>> int qemu_savevm_state_iterate(QEMUFile *f, bool postcopy)
>>
>> This is to serialize savevm_state.handlers (or in other words devices).
> 
> I've lost all context on this question in the interim, but I think this
> highlights my question.  We use pending_bytes to know how close we are
> to the end of the stream and data_size to iterate each transaction
> within that stream.  So how does data_size == 0 indicate we've
> completed the current phase?  It seems like pending_bytes should
> indicate that.  Thanks,
> 

Fixing this by adding a read on pending_bytes if its 0 and return 
accordingly.
     if (migration->pending_bytes == 0) {
         ret = vfio_update_pending(vbasedev);
         if (ret) {
             return ret;
         }

         if (migration->pending_bytes == 0) {
             /* indicates data finished, goto complete phase */
             return 1;
         }
     }

Thanks,
Kirti