Re: [PATCH v2] virtio-iommu: Use qemu_real_host_page_mask as default page_size_mask

[Eric Auger]

On 2/13/24 12:07, Michael S. Tsirkin wrote:
> On Tue, Feb 13, 2024 at 11:32:13AM +0100, Eric Auger wrote:
>> Hi Michael,
>>
>> On 2/13/24 10:43, Michael S. Tsirkin wrote:
>>> On Wed, Jan 17, 2024 at 02:20:39PM +0100, Eric Auger wrote:
>>>> We used to set default page_size_mask to qemu_target_page_mask() but
>>>> with VFIO assignment it makes more sense to use the actual host page mask
>>>> instead.
>>>>
>>>> So from now on qemu_real_host_page_mask() will be used as a default.
>>>> To be able to migrate older code, we increase the vmstat version_id
>>>> to 3 and if an older incoming v2 stream is detected we set the previous
>>>> default value.
>>>>
>>>> The new default is well adapted to configs where host and guest have
>>>> the same page size. This allows to fix hotplugging VFIO devices on a
>>>> 64kB guest and a 64kB host. This test case has been failing before
>>>> and even crashing qemu with hw_error("vfio: DMA mapping failed,
>>>> unable to continue") in VFIO common). Indeed the hot-attached VFIO
>>>> device would call memory_region_iommu_set_page_size_mask with 64kB
>>>> mask whereas after the granule was frozen to 4kB on machine init done.
>>>> Now this works. However the new default will prevent 4kB guest on
>>>> 64kB host because the granule will be set to 64kB which would be
>>>> larger than the guest page size. In that situation, the virtio-iommu
>>>> driver fails on viommu_domain_finalise() with
>>>> "granule 0x10000 larger than system page size 0x1000".
>>>>
>>>> The current limitation of global granule in the virtio-iommu
>>>> should be removed and turned into per domain granule. But
>>>> until we get this upgraded, this new default is probably
>>>> better because I don't think anyone is currently interested in
>>>> running a 4kB page size guest with virtio-iommu on a 64kB host.
>>>> However supporting 64kB guest on 64kB host with virtio-iommu and
>>>> VFIO looks a more important feature.
>>>>
>>>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>>>> Tested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
>>>> Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
>>> What about migration compatibility? In particular, cross-version one?
>>> Don't we need compat machinery for this?
>> See below
>>>> ---
>>>>
>>>> v1 -> v2:
>>>> - fixed 2 typos in the commit msg and added Jean's R-b and T-b
>>>> ---
>>>>  hw/virtio/virtio-iommu.c | 7 +++++--
>>>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/hw/virtio/virtio-iommu.c b/hw/virtio/virtio-iommu.c
>>>> index 8a4bd933c6..ec2ba11d1d 100644
>>>> --- a/hw/virtio/virtio-iommu.c
>>>> +++ b/hw/virtio/virtio-iommu.c
>>>> @@ -1313,7 +1313,7 @@ static void virtio_iommu_device_realize(DeviceState 
>>>> *dev, Error **errp)
>>>>       * in vfio realize
>>>>       */
>>>>      s->config.bypass = s->boot_bypass;
>>>> -    s->config.page_size_mask = qemu_target_page_mask();
>>>> +    s->config.page_size_mask = qemu_real_host_page_mask();
>>>>      s->config.input_range.end = UINT64_MAX;
>>>>      s->config.domain_range.end = UINT32_MAX;
>>>>      s->config.probe_size = VIOMMU_PROBE_SIZE;
>>>> @@ -1491,13 +1491,16 @@ static int iommu_post_load(void *opaque, int 
>>>> version_id)
>>>>       * still correct.
>>>>       */
>>>>      virtio_iommu_switch_address_space_all(s);
>>>> +    if (version_id <= 2) {
>>>> +        s->config.page_size_mask = qemu_target_page_mask();
>> I tested migration from v2 -> v3 and the above code is overriding the
>> new default by the older one.
>>
>> Do you have an other concern?
>>
>> Thanks
>>
>> Eric
>
>
>>>> +    }
>>>>      return 0;
>>>>  }
>>>>  
>>>>  static const VMStateDescription vmstate_virtio_iommu_device = {
>>>>      .name = "virtio-iommu-device",
>>>>      .minimum_version_id = 2,
>>>> -    .version_id = 2,
>>>> +    .version_id = 3,
>>>>      .post_load = iommu_post_load,
>>>>      .fields = (const VMStateField[]) {
>>>>          VMSTATE_GTREE_DIRECT_KEY_V(domains, VirtIOIOMMU, 2,
> Oh I missed the version change. But then migration  to older version
> is completely broken isn't it? Old qemu can not handle version_id 3 at
> all.
Indeed, I considered migrating backyard was not that much important. Do
you consider this is mandated?

Eric
>
> Generally, compat machinery is nicer than the old version hacks.
>
>
>>>> -- 
>>>> 2.41.0
>