> Am 28.02.2020 um 21:49 schrieb Peter Xu <pet...@redhat.com>:
>
> On Fri, Feb 28, 2020 at 03:19:45PM -0500, David Hildenbrand wrote:
>>
>>
>>>> Am 28.02.2020 um 20:55 schrieb Peter Xu <pet...@redhat.com>:
>>>
>>> On Thu, Feb 27, 2020 at 11:12:02AM +0100, David Hildenbrand wrote:
>>>> +static void qemu_vfio_dma_map_resize(QEMUVFIOState *s, void *host,
>>>> + size_t old_size, size_t new_size)
>>>> +{
>>>> + IOVAMapping *m;
>>>> + int index = 0;
>>>> +
>>>> + qemu_mutex_lock(&s->lock);
>>>> + m = qemu_vfio_find_mapping(s, host, &index);
>>>> + if (!m) {
>>>> + return;
>>>> + }
>>>> + assert(m->size == old_size);
>>>> +
>>>> + /* Note: Not atomic - we need a new ioctl for that. */
>>>> + qemu_vfio_undo_mapping(s, m->iova, m->size);
>>>> + qemu_vfio_do_mapping(s, host, m->iova, new_size);
>>>
>>> Another way to ask my previous question 1 (in the other reply): Can we
>>> simply map/unmap the extra, while keep the shared untouched?
>>
>> As far as I understand the kernel implementation, unfortunately no. You
>> might be able to grow (by mapping the delta), but shrinking is not possible
>> AFAIR. And I *think* with many resizes, there could be an issue if I
>> remember correctly.
>
> Ah right (and I think the same thing will happen to vfio-pci during
> memory_region_set_size()). Then please double confirm that virtio-mem
> is disabled for both vfio-helper users and vfio-pci. Also, would you
Yes, will double check, should have taken care of both.
> mind comment above the unmap+map with more information? E.g.:
>
Sure, will add - thanks!
> For now, we must unmap the whole mapped range first and remap with
> the new size. The reason is that VFIO_IOMMU_UNMAP_DMA might fail
> with partial unmap of previous mappings, so even we can add extra
> new mappings to extend the old range, however we still won't able to
> always success on shrinking memories. The side effect is that it's
> never safe to do resizing during VM execution.
>
> Or something better. With an enriched comment, I think it's fine:
>
> Reviewed-by: Peter Xu <pet...@redhat.com>
>
Thanks Peter! Have a nice weekend!
