Re: [PATCH 0/2] virtio: decouple protected guest RAM form VIRTIO_F_IOMMU_PLATFORM

[Jason Wang <jasowang@redhat.com>]

On 2020/2/24 下午9:40, Michael S. Tsirkin wrote:
>> Subject: [PATCH] vhost: do not set VIRTIO_F_IOMMU_PLATFORM when IOMMU is not
>>   used
>>
>> We enable device IOTLB unconditionally when VIRTIO_F_IOMMU_PLATFORM is
>> negotiated. This lead unnecessary IOTLB miss/update transactions when
>> IOMMU is used. This patch fixes this.
>>
>> Signed-off-by: Jason Wang<jasowang@redhat.com>
>> ---
>>   hw/net/virtio-net.c | 3 +++
>>   hw/virtio/vhost.c   | 4 +---
>>   2 files changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
>> index 3627bb1717..0d50e8bd34 100644
>> --- a/hw/net/virtio-net.c
>> +++ b/hw/net/virtio-net.c
>> @@ -879,6 +879,9 @@ static void virtio_net_set_features(VirtIODevice *vdev, uint64_t features)
>>           virtio_net_apply_guest_offloads(n);
>>       }
>>   
>> +    if (vdev->dma_as == &address_space_memory)
>> +        features &= ~(1ULL << VIRTIO_F_IOMMU_PLATFORM);
>> +
>>       for (i = 0;  i < n->max_queues; i++) {
>>           NetClientState *nc = qemu_get_subqueue(n->nic, i);
> This pokes at acked features. I think they are also
> guest visible ...


It's the acked features of vhost device, so I guess not?

E.g virtio_set_features_nocheck() did:

     val &= vdev->host_features;
     if (k->set_features) {
         k->set_features(vdev, val);
     }
     vdev->guest_features = val;

Thanks

>