Virtualization - Jun 2021 - [PATCH v1 1/8] virtio: Force only split mode with protected guest

? 2021/6/3 ??9:48, Andi Kleen ??:
>
>> So we will see huge performance regression without indirect 
>> descriptor. We need to consider to address this.
>
> A regression would be when some existing case would be slower.
>
> That's not the case because the behavior for the existing cases does 
> not change.
>
> Anyways when there are performance problems they can be addressed, but 
> first is to make it secure.

I agree, but I want to know why indirect descriptor needs to be 
disabled. The table can't be wrote by the device since it's not coherent
swiotlb mapping.

Thanks

>
> -Andi
>
>
>>
>> Thanks
>>
>>
>>> ????????????? break;
>>> ????????? case VIRTIO_RING_F_EVENT_IDX:
>>> ????????????? break;
>>> @@ -2231,9 +2240,12 @@ void vring_transport_features(struct 
>>> virtio_device *vdev)
>>> ????????? case VIRTIO_F_ACCESS_PLATFORM:
>>> ????????????? break;
>>> ????????? case VIRTIO_F_RING_PACKED:
>>> +??????????? if (protected_guest_has(VM_MEM_ENCRYPT))
>>> +??????????????? goto clear;
>>> ????????????? break;
>>> ????????? case VIRTIO_F_ORDER_PLATFORM:
>>> ????????????? break;
>>> +??????? clear:
>>> ????????? default:
>>> ????????????? /* We don't understand this bit. */
>>> ????????????? __virtio_clear_bit(vdev, i);
>>
>

>
> I agree, but I want to know why indirect descriptor needs to be 
> disabled. The table can't be wrote by the device since it's not 
> coherent swiotlb mapping.
I had all kinds of problems with uninitialized entries in the indirect 
table. So I gave up on it and concluded it would be too difficult to secure.


-Andi