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

[Eric Auger]

Hi Jean,

On 1/16/24 13:53, Jean-Philippe Brucker wrote:
> On Thu, Dec 21, 2023 at 08:45:05AM -0500, 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.
> I guess TARGET_PAGE_MASK is always 4kB on arm64 CPUs, since it's the
> smallest supported and the guest configures its page size at runtime.
correct
> Even if QEMU's software IOMMU can deal with any page size, VFIO can't so
> passing the host page size seems more accurate than forcing a value of
> 4kB.
that's my guess too
>
>> 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 the viommu_domain_finalise() with
>> "granule 0x10000 larger than system page zie 0x1000".
> "size"
> (it could matter if someone searches for this message later)
sure I will correct it
>
>> 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>
> So to summarize the configurations that work for hotplug (tested with QEMU
> system emulation with SMMU + QEMU VMM with virtio-iommu):
>
>  Host | Guest | virtio-net | IGB passthrough
>   4k  | 4k    | Y          | Y
>   64k | 64k   | Y          | N -> Y (fixed by this patch)
>   64k | 4k    | Y -> N     | N
>   4k  | 64k   | Y          | Y
That table is correct
>
> The change is a reasonable trade-off in my opinion. It fixes the more common
> 64k on 64k case, and for 4k on 64k, the error is now contained to the
> guest and made clear ("granule 0x10000 larger than system page size
> 0x1000") instead of crashing the VMM. A guest OS now discovers that the
> host needs DMA buffers aligned on 64k and could actually support this case
> (but Linux won't because it can't control the origin of all DMA buffers).
> Later, support for page tables will enable 4k on 64k for all devices.
>
> Tested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>

Many thanks!

Eric
>
>> ---
>>  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 9d463efc52..b77e3644ea 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();
>> +    }
>>      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 = (VMStateField[]) {
>>          VMSTATE_GTREE_DIRECT_KEY_V(domains, VirtIOIOMMU, 2,
>> -- 
>> 2.27.0
>>