linux-kernel - Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

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Message-Id: <87womn8inf.fsf@morokweng.localdomain>
Date:   Tue, 29 Jan 2019 15:42:44 -0200
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     virtualization@...ts.linux-foundation.org
Cc:     linuxppc-dev@...ts.ozlabs.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Christoph Hellwig <hch@....de>,
        David Gibson <david@...son.dropbear.id.au>,
        Alexey Kardashevskiy <aik@...ux.ibm.com>,
        Paul Mackerras <paulus@...abs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Ram Pai <linuxram@...ibm.com>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted


Fixing address of powerpc mailing list.

Thiago Jung Bauermann <bauerman@...ux.ibm.com> writes:

> Hello,
>
> With Christoph's rework of the DMA API that recently landed, the patch
> below is the only change needed in virtio to make it work in a POWER
> secure guest under the ultravisor.
>
> The other change we need (making sure the device's dma_map_ops is NULL
> so that the dma-direct/swiotlb code is used) can be made in
> powerpc-specific code.
>
> Of course, I also have patches (soon to be posted as RFC) which hook up
> <linux/mem_encrypt.h> to the powerpc secure guest support code.
>
> What do you think?
>
> From d0629a36a75c678b4a72b853f8f7f8c17eedd6b3 Mon Sep 17 00:00:00 2001
> From: Thiago Jung Bauermann <bauerman@...ux.ibm.com>
> Date: Thu, 24 Jan 2019 22:08:02 -0200
> Subject: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted
>
> The host can't access the guest memory when it's encrypted, so using
> regular memory pages for the ring isn't an option. Go through the DMA API.
>
> Signed-off-by: Thiago Jung Bauermann <bauerman@...ux.ibm.com>
> ---
>  drivers/virtio/virtio_ring.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index cd7e755484e3..321a27075380 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -259,8 +259,11 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
>  	 * not work without an even larger kludge.  Instead, enable
>  	 * the DMA API if we're a Xen guest, which at least allows
>  	 * all of the sensible Xen configurations to work correctly.
> +	 *
> +	 * Also, if guest memory is encrypted the host can't access
> +	 * it directly. In this case, we'll need to use the DMA API.
>  	 */
> -	if (xen_domain())
> +	if (xen_domain() || sev_active())
>  		return true;
>
>  	return false;


-- 
Thiago Jung Bauermann
IBM Linux Technology Center