Re: [PATCH v1 1/8] virtio: Force only split mode with protected guest

From: Jason Wang <jasowang@redhat.com>
To: Andi Kleen <ak@linux.intel.com>, mst@redhat.com
Cc: virtualization@lists.linux-foundation.org, hch@lst.de,
	m.szyprowski@samsung.com, robin.murphy@arm.com,
	iommu@lists.linux-foundation.org, x86@kernel.org,
	sathyanarayanan.kuppuswamy@linux.intel.com, jpoimboe@redhat.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 1/8] virtio: Force only split mode with protected guest
Date: Thu, 3 Jun 2021 10:32:40 +0800	[thread overview]
Message-ID: <1c08bc42-7448-351e-78bf-fcf68d2b2561@redhat.com> (raw)
In-Reply-To: <09e17c7f-ce51-1a46-72c4-12223bee4e3a@linux.intel.com>

在 2021/6/3 上午9:48, Andi Kleen 写道:
>
>> 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);
>>
>