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Subject: Re: [PATCH QEMU v25 08/17] vfio: Add save state functions to
 SaveVMHandlers
To: Alex Williamson <alex.williamson@redhat.com>
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From: Kirti Wankhede <kwankhede@nvidia.com>
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Date: Wed, 24 Jun 2020 02:04:24 +0530
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On 6/23/2020 4:20 AM, Alex Williamson wrote:
> On Sun, 21 Jun 2020 01:51:17 +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           | 283 ++++++++++++++++++++++++++++++++++++++++++
>>   hw/vfio/trace-events          |   6 +
>>   include/hw/vfio/vfio-common.h |   1 +
>>   3 files changed, 290 insertions(+)
>>
>> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
>> index 133bb5b1b3b2..ef1150c1ff02 100644
>> --- a/hw/vfio/migration.c
>> +++ b/hw/vfio/migration.c
>> @@ -140,6 +140,168 @@ static int vfio_migration_set_state(VFIODevice *vbasedev, uint32_t mask,
>>       return 0;
>>   }
>>   
>> +static void *get_data_section_size(VFIORegion *region, uint64_t data_offset,
>> +                                   uint64_t data_size, uint64_t *size)
>> +{
>> +    void *ptr = NULL;
>> +    int i;
>> +
>> +    if (!region->mmaps) {
>> +        *size = data_size;
>> +        return ptr;
>> +    }
>> +
>> +    /* check if data_offset in within sparse mmap areas */
>> +    for (i = 0; i < region->nr_mmaps; i++) {
>> +        VFIOMmap *map = region->mmaps + i;
>> +
>> +        if ((data_offset >= map->offset) &&
>> +            (data_offset < map->offset + map->size)) {
>> +            ptr = map->mmap + data_offset - map->offset;
>> +
>> +            if (data_offset + data_size <= map->offset + map->size) {
>> +                *size = data_size;
>> +            } else {
>> +                *size = map->offset + map->size - data_offset;
>> +            }
> 
> Ultimately we take whichever result is smaller, so we could just use:
> 
> *size = MIN(data_size, map->offset + map->size - data_offset);
> 
>> +            break;
>> +        }
>> +    }
>> +
>> +    if (!ptr) {
>> +        uint64_t limit = 0;
>> +
>> +        /*
>> +         * data_offset is not within sparse mmap areas, find size of non-mapped
>> +         * area. Check through all list since region->mmaps list is not sorted.
>> +         */
>> +        for (i = 0; i < region->nr_mmaps; i++) {
>> +            VFIOMmap *map = region->mmaps + i;
>> +
>> +            if ((data_offset < map->offset) &&
>> +                (!limit || limit > map->offset)) {
>> +                limit = map->offset;
>> +            }
> 
> We could have done this in an else branch of the previous loop to avoid
> walking the entries twice.
> 

Ok. updating with above 2 changes.

>> +        }
>> +
>> +        *size = limit ? limit - data_offset : data_size;
>> +    }
>> +    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, size;
>> +    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;
>> +    }
>> +
>> +    trace_vfio_save_buffer(vbasedev->name, data_offset, data_size,
>> +                           migration->pending_bytes);
>> +
>> +    qemu_put_be64(f, data_size);
>> +    size = data_size;
>> +
>> +    while (size) {
>> +        void *buf = NULL;
>> +        bool buffer_mmaped;
>> +        uint64_t sec_size;
>> +
>> +        buf = get_data_section_size(region, data_offset, size, &sec_size);
>> +
>> +        buffer_mmaped = (buf != NULL);
>> +
>> +        if (!buffer_mmaped) {
>> +            buf = g_try_malloc(sec_size);
>> +            if (!buf) {
>> +                error_report("%s: Error allocating buffer ", __func__);
>> +                return -ENOMEM;
>> +            }
>> +
>> +            ret = pread(vbasedev->fd, buf, sec_size,
>> +                        region->fd_offset + data_offset);
> 
> Is the trade-off to allocate this buffer worth it?  I'd be tempted to
> iterate with a basic data type here to avoid what could potentially be
> a large memory allocation above.  It feels a little more robust, if not
> perhaps as fast, but I this will mostly be a fallback or only cover
> small ranges in normal operation.  Of course the data stream needs to
> be compatible either way we retrieve it.
> 

What should be basic data type here, u8, u16, u32, u64? We don't know at 
what granularity vendor driver is writing, then I thnk we have to go 
with smallest u8, right?


>> +            if (ret != sec_size) {
>> +                error_report("%s: Failed to get migration data %d",
>> +                             vbasedev->name, ret);
>> +                g_free(buf);
>> +                return -EINVAL;
>> +            }
>> +        }
>> +
>> +        qemu_put_buffer(f, buf, sec_size);
>> +
>> +        if (!buffer_mmaped) {
>> +            g_free(buf);
>> +        }
>> +        size -= sec_size;
>> +        data_offset += sec_size;
>> +    }
>> +
>> +    ret = qemu_file_get_error(f);
>> +    if (ret) {
>> +        return ret;
>> +    }
>> +
>> +    return data_size;
> 
> This function returns int, data_size is uint64_t.  Thanks,
> 

Yes, returns for this function:
< 0 => error
==0 => no more data to save
data_size => amount of data saved in this function.

Thanks,
Kirti