RE: [PATCH v5 3/3] virtio-iommu: Change the default granule to the host page size

[Duan, Zhenzhong]

Hi Eric,

>-----Original Message-----
>From: Eric Auger <eric.auger@redhat.com>
>Subject: [PATCH v5 3/3] virtio-iommu: Change the default granule to the
>host page size
>
>We used to set the default granule to 4KB but with VFIO assignment
>it makes more sense to use the actual host page size.
>
>Indeed when hotplugging a VFIO device protected by a virtio-iommu
>on a 64kB/64kB host/guest config, we current get a qemu crash:
>
>"vfio: DMA mapping failed, unable to continue"
>
>This is due to the hot-attached VFIO device calling
>memory_region_iommu_set_page_size_mask() with 64kB granule
>whereas the virtio-iommu granule was already frozen to 4KB on
>machine init done.
>
>Set the granule property to "host" and introduce a new compat.
>The page size mask used before 9.0 was qemu_target_page_mask().
>Since the virtio-iommu currently only supports x86_64 and aarch64,
>this matched a 4KB granule.
>
>Note that 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".
>
>In that case the workaround is to request 4K granule.
>
>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>
>Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>

Thanks
Zhenzhong

>
>---
>
>v4 -> v5
>- use low case, mandated by the jason qapi
>---
> hw/core/machine.c        | 1 +
> hw/virtio/virtio-iommu.c | 2 +-
> 2 files changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/hw/core/machine.c b/hw/core/machine.c
>index 70ac96954c..56f38b6579 100644
>--- a/hw/core/machine.c
>+++ b/hw/core/machine.c
>@@ -35,6 +35,7 @@
>
> GlobalProperty hw_compat_8_2[] = {
>     { TYPE_VIRTIO_IOMMU_PCI, "aw-bits", "64" },
>+    { TYPE_VIRTIO_IOMMU_PCI, "granule", "4k" },
> };
> const size_t hw_compat_8_2_len = G_N_ELEMENTS(hw_compat_8_2);
>
>diff --git a/hw/virtio/virtio-iommu.c b/hw/virtio/virtio-iommu.c
>index 33e0520bc8..6831446e29 100644
>--- a/hw/virtio/virtio-iommu.c
>+++ b/hw/virtio/virtio-iommu.c
>@@ -1548,7 +1548,7 @@ static Property virtio_iommu_properties[] = {
>     DEFINE_PROP_BOOL("boot-bypass", VirtIOIOMMU, boot_bypass, true),
>     DEFINE_PROP_UINT8("aw-bits", VirtIOIOMMU, aw_bits, 0),
>     DEFINE_PROP_GRANULE_MODE("granule", VirtIOIOMMU, granule_mode,
>-                             GRANULE_MODE_4K),
>+                             GRANULE_MODE_HOST),
>     DEFINE_PROP_END_OF_LIST(),
> };
>
>--
>2.41.0