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

["Michael S. Tsirkin" <mst@redhat.com>]

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.


Generally, compat machinery is nicer than the old version hacks.


> >> -- 
> >> 2.41.0