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

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Date:   Mon, 20 May 2019 09:16:57 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Thiago Jung Bauermann <bauerman@...ux.ibm.com>
Cc:     virtualization@...ts.linux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, 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>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Michael Roth <mdroth@...ux.vnet.ibm.com>,
        Mike Anderson <andmike@...ux.ibm.com>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

On Wed, Apr 17, 2019 at 06:42:00PM -0300, Thiago Jung Bauermann wrote:
> I rephrased it in terms of address translation. What do you think of
> this version? The flag name is slightly different too:
> 
> 
> VIRTIO_F_ACCESS_PLATFORM_NO_TRANSLATION This feature has the same
>     meaning as VIRTIO_F_ACCESS_PLATFORM both when set and when not set,
>     with the exception that address translation is guaranteed to be
>     unnecessary when accessing memory addresses supplied to the device
>     by the driver. Which is to say, the device will always use physical
>     addresses matching addresses used by the driver (typically meaning
>     physical addresses used by the CPU) and not translated further. This
>     flag should be set by the guest if offered, but to allow for
>     backward-compatibility device implementations allow for it to be
>     left unset by the guest. It is an error to set both this flag and
>     VIRTIO_F_ACCESS_PLATFORM.


OK so VIRTIO_F_ACCESS_PLATFORM is designed to allow unpriveledged
drivers. This is why devices fail when it's not negotiated.

This confuses me.
If driver is unpriveledged then what happens with this flag?
It can supply any address it wants. Will that corrupt kernel
memory?

-- 
MST