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

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Message-Id: <20190811064621.GB5964@ram.ibm.com>
Date:   Sat, 10 Aug 2019 23:46:21 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linuxppc-devel@...ts.ozlabs.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        David Gibson <david@...son.dropbear.id.au>,
        Alexey Kardashevskiy <aik@...ux.ibm.com>,
        Paul Mackerras <paulus@...abs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: RE: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

On Sun, Aug 11, 2019 at 07:56:07AM +0200, Christoph Hellwig wrote:
> sev_active() is gone now in linux-next, at least as a global API.
> 
> And once again this is entirely going in the wrong direction.  The only
> way using the DMA API is going to work at all is if the device is ready
> for it.  So we need a flag on the virtio device, exposed by the
> hypervisor (or hardware for hw virtio devices) that says:  hey, I'm real,
> don't take a shortcut.
> 
> And that means on power and s390 qemu will always have to set thos if
> you want to be ready for the ultravisor and co games.  It's not like we
> haven't been through this a few times before, have we?


We have been through this so many times, but I dont think, we ever
understood each other.   I have a fundamental question, the answer to
which was never clear. Here it is...

If the hypervisor (hardware for hw virtio devices) does not mandate a
DMA API, why is it illegal for the driver to request, special handling
of its i/o buffers? Why are we associating this special handling to
always mean, some DMA address translation? Can't there be 
any other kind of special handling needs, that has nothing to do with
DMA address translation?


-- 
Ram Pai