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Re: [for-5.2 v4 10/10] s390: Recognize host-trust-limitation option

From: Halil Pasic
Subject: Re: [for-5.2 v4 10/10] s390: Recognize host-trust-limitation option
Date: Mon, 7 Sep 2020 17:22:53 +0200

On Fri, 24 Jul 2020 12:57:44 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> At least some s390 cpu models support "Protected Virtualization" (PV),
> a mechanism to protect guests from eavesdropping by a compromised
> hypervisor.
> This is similar in function to other mechanisms like AMD's SEV and
> POWER's PEF, which are controlled bythe "host-trust-limitation"
> machine option.  s390 is a slightly special case, because we already
> supported PV, simply by using a CPU model with the required feature
> To integrate this with the option used by other platforms, we
> implement the following compromise:
>  - When the host-trust-limitation option is set, s390 will recognize
>    it, verify that the CPU can support PV (failing if not) and set
>    virtio default options necessary for encrypted or protected guests,
>    as on other platforms.  i.e. if host-trust-limitation is set, we
>    will either create a guest capable of entering PV mode, or fail
>    outright

Shouldn't we also fail outright if the virtio features are not PV
compatible (invalid configuration)?

I would like to see something like follows as a part of this series.
From: Halil Pasic <pasic@linux.ibm.com>
Date: Mon, 7 Sep 2020 15:00:17 +0200
Subject: [PATCH] virtio: handle host trust limitation

If host_trust_limitation_enabled() returns true, then emulated virtio
devices must offer VIRTIO_F_ACCESS_PLATFORM, because the device is not
capable of accessing all of the guest memory. Otherwise we are in
violation of the virtio specification.

Let's fail realize if we detect that VIRTIO_F_ACCESS_PLATFORM feature is
obligatory but missing.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
 hw/virtio/virtio.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 5bd2a2f621..19b4b0a37a 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -27,6 +27,7 @@
 #include "hw/virtio/virtio-access.h"
 #include "sysemu/dma.h"
 #include "sysemu/runstate.h"
+#include "exec/host-trust-limitation.h"
  * The alignment to use between consumer and producer parts of vring.
@@ -3618,6 +3619,12 @@ static void virtio_device_realize(DeviceState *dev, 
Error **errp)
     /* Devices should either use vmsd or the load/save methods */
     assert(!vdc->vmsd || !vdc->load);
+    if (host_trust_limitation_enabled(MACHINE(qdev_get_machine()))
+        && !virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
+        error_setg(&err, "devices without VIRTIO_F_ACCESS_PLATFORM are not 
compatible with host trust imitation");
+        error_propagate(errp, err);
+        return;
+    }
     if (vdc->realize != NULL) {
         vdc->realize(dev, &err);
         if (err != NULL) {

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