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Subject: Re: [PATCH v1 1/8] virtio: Force only split mode with protected guest
To:     Jason Wang <jasowang@redhat.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
References: <20210603004133.4079390-1-ak@linux.intel.com>
 <20210603004133.4079390-2-ak@linux.intel.com>
 <28c8302b-6833-10b4-c0eb-67456e7c4069@redhat.com>
From:   Andi Kleen <ak@linux.intel.com>
Message-ID: <09e17c7f-ce51-1a46-72c4-12223bee4e3a@linux.intel.com>
Date:   Wed, 2 Jun 2021 18:48:11 -0700
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> 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.

-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);
>