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

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Date:   Sun, 11 Aug 2019 04:44:17 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Ram Pai <linuxram@...ibm.com>
Cc:     Christoph Hellwig <hch@....de>,
        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 Sat, Aug 10, 2019 at 11:46:21PM -0700, Ram Pai wrote:
> 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?

I think the answer to that is, extend the DMA API to cover that special
need then. And that's exactly what dma_addr_is_phys_addr is trying to
do.

> 
> -- 
> Ram Pai