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

On Sat, Aug 10, 2019 at 03:07:02PM -0700, Ram Pai wrote:

On Sat, Aug 10, 2019 at 02:57:17PM -0400, Michael S. Tsirkin wrote:

quoted

On Tue, Jan 29, 2019 at 03:08:12PM -0200, Thiago Jung Bauermann wrote:

quoted

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?

quoted

From d0629a36a75c678b4a72b853f8f7f8c17eedd6b3 Mon Sep 17 00:00:00 2001

From: Thiago Jung Bauermann <redacted>
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 <redacted>
---
 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;

So I gave this lots of thought, and I'm coming round to
basically accepting something very similar to this patch.

But not exactly like this :).

Let's see what are the requirements.

If

1. We do not trust the device (so we want to use a bounce buffer with it)
2. DMA address is also a physical address of a buffer

then we should use DMA API with virtio.


sev_active() above is one way to put (1).  I can't say I love it but
it's tolerable.


But we also want promise from DMA API about 2.


Without promise 2 we simply can't use DMA API with a legacy device.


Otherwise, on a SEV system with an IOMMU which isn't 1:1
and with a virtio device without ACCESS_PLATFORM, we are trying
to pass a virtual address, and devices without ACCESS_PLATFORM
can only access CPU physical addresses.

So something like:

dma_addr_is_phys_addr?

On our Secure pseries platform,  dma address is physical address and this
proposal will help us, use DMA API. 

On our normal pseries platform, dma address is physical address too.
But we do not necessarily need to use the DMA API.  We can use the DMA
API, but our handlers will do the same thing, the generic virtio handlers
would do. If there is an opt-out option; even when dma addr is same as
physical addr, than there will be less code duplication.

Would something like this be better.

(dma_addr_is_phys_addr  && arch_want_to_use_dma_api()) ?


RP

I think sev_active() is an OK replacement for arch_want_to_use_dma_api.
So just the addition of dma_addr_is_phys_addr would be enough.

quoted

-- 
MST

-- 
Ram Pai