linux-kernel - Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is protected

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Date:   Thu, 20 Feb 2020 17:13:09 +0100
From:   Christoph Hellwig <hch@....de>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>, linux-s390@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Viktor Mihajlovski <mihajlov@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Ram Pai <linuxram@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        David Gibson <david@...son.dropbear.id.au>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Michael Mueller <mimu@...ux.ibm.com>
Subject: Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is
 protected

On Thu, Feb 20, 2020 at 05:06:06PM +0100, Halil Pasic wrote:
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 867c7ebd3f10..fafc8f924955 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -243,6 +243,9 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
>  	if (!virtio_has_iommu_quirk(vdev))
>  		return true;
>  
> +	if (force_dma_unencrypted(&vdev->dev))
> +		return true;

Hell no.  This is a detail of the platform DMA direct implementation.
Drivers have no business looking at this flag, and virtio finally needs
to be fixed to use the DMA API properly for everything but legacy devices.

No amount of desparate hacks is going to fix that fundamental problem,
and I'm starting to get really sick and tired of all the crap patches
published in this area.