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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jan 30, 2019 at 10:24:01AM +0800, Jason Wang wrote:
> 
> On 2019/1/30 上午3:02, Michael S. Tsirkin wrote:
> > On Tue, Jan 29, 2019 at 03:42:44PM -0200, Thiago Jung Bauermann wrote:
> > > Fixing address of powerpc mailing list.
> > > 
> > > Thiago Jung Bauermann <bauerman@linux.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@linux.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@linux.ibm.com>
> > Well I think this will come back to bite us (witness xen which is now
> > reworking precisely this path - but at least they aren't to blame, xen
> > came before ACCESS_PLATFORM).
> > 
> > I also still think the right thing would have been to set
> > ACCESS_PLATFORM for all systems where device can't access all memory.
> > 
> > But I also think I don't have the energy to argue about power secure
> > guest anymore.  So be it for power secure guest since the involved
> > engineers disagree with me.  Hey I've been wrong in the past ;).
> > 
> > But the name "sev_active" makes me scared because at least AMD guys who
> > were doing the sensible thing and setting ACCESS_PLATFORM (unless I'm
> > wrong? I reemember distinctly that's so) will likely be affected too.
> > We don't want that.
> > 
> > So let's find a way to make sure it's just power secure guest for now
> > pls.
> > 
> > I also think we should add a dma_api near features under virtio_device
> > such that these hacks can move off data path.
> 
> 
> Anyway the current Xen code is conflict with spec which said:
> 
> "If this feature bit is set to 0, then the device has same access to memory
> addresses supplied to it as the driver has. In particular, 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, and can access any address supplied to it by the driver. When
> clear, this overrides any platform-specific description of whether device
> access is limited or translated in any way, e.g. whether an IOMMU may be
> present. "
> 
> I wonder how much value that the above description can give us. It's kind of
> odd that the behavior of "when the feature is not negotiated" is described
> in the spec.

Hmm what's odd about it? We need to describe the behaviour is all cases.

> Personally I think we can remove the above and then we can
> switch to use DMA API unconditionally in guest driver. It may have single
> digit regression probably, we can try to overcome it.
> 
> Thanks

This has been discussed ad nauseum. virtio is all about compatibility.
Losing a couple of lines of code isn't worth breaking working setups.
People that want "just use DMA API no tricks" now have the option.
Setting a flag in a feature bit map is literally a single line
of code in the hypervisor. So stop pushing for breaking working
legacy setups and just fix it in the right place.

> 
> > 
> > By the way could you please respond about virtio-iommu and
> > why there's no support for ACCESS_PLATFORM on power?
> > 
> > I have Cc'd you on these discussions.
> > 
> > 
> > Thanks!
> > 
> > 
> > > > ---
> > > >   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