Re: [PATCH QEMU v25 08/17] vfio: Add save state functions to SaveVMHandlers

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

On Wed, 24 Jun 2020 02:04:24 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> 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?

That'd be a little on the ridiculous side.  We could make a helper like
in vfio_pci_rdwr that reads at the largest aligned size up to u64.

> >> +            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.

So when data_size exceeds MAX_UINT, the return value goes negative...

Thanks,
Alex