Re: [PATCH v5 07/18] s390x: protvirt: Inhibit balloon when switching to protected mode

[David Hildenbrand <david@redhat.com>]

On 27.02.20 13:24, Halil Pasic wrote:
> On Wed, 26 Feb 2020 16:11:03 +0100
> Janosch Frank <frankja@linux.ibm.com> wrote:
> 
>> On 2/26/20 3:59 PM, David Hildenbrand wrote:
>>> On 26.02.20 13:20, Janosch Frank wrote:
>>>> Ballooning in protected VMs can only be done when the guest shares the
>>>> pages it gives to the host. Hence, until we have a solution for this
>>>> in the guest kernel, we inhibit ballooning when switching into
>>>> protected mode and reverse that once we move out of it.
>>>
>>> I don't understand what you mean here, sorry. zapping a page will mean
>>> that a fresh one will be faulted in when accessed. And AFAIK, that means
>>> it will be encrypted again when needed.
>>
>> Yes, as soon as the host alters non-shared memory we'll run into
>> integrity issues.
>>
>>
>> I've been talking to Halil after I sent this out and it looks like we'll
>> rather try to automatically enable the IOMMU for all devices when
>> switching into protected mode. He said that if the IOMMU is set the
>> balloon code will do an early exit on feature negotiation.
>>
> 
> I have a discussion starter RFC for you.
> 
> --------------------------8<----------------------------------------------
> From: Halil Pasic <pasic@linux.ibm.com>
> Date: Wed, 26 Feb 2020 16:48:21 +0100
> Subject: [RFC PATCH 1/1] virtio-ccw: auto-manage VIRTIO_F_IOMMU_PLATFORM
> 
> The virtio specification tells that the device is to present
> VIRTIO_F_ACCESS_PLATFORM (a.k.a. VIRTIO_F_IOMMU_PLATFORM) when the
> device "can only access certain memory addresses with said access
> specified and/or granted by the platform". This is the case for a
> protected VM, as the device can access only memory addresses that are in
> pages that are currently shared (only the guest can share/unsare its
> page).
> 
> No VM however starts out as a protected VM, but some VMs may be
> converted to protected VMs if the guest decides so.
> 
> Making the end user explicitly manage the VIRTIO_F_ACCESS_PLATFORM via
> the property iommu_on is a minor disaster. If the correctness of the
> paravirtualized virtio devices depends (and thus in a sense the
> correctness of the hypervisor), then the hypervisor should have the last
> word about whether VIRTIO_F_ACCESS_PLATFORM is to be presented or not.
> 
> Let's manage the VIRTIO_F_ACCESS_PLATFORM virtio feature automatically
> for virtio-ccw devices, so that we set it before we start the protected
> configuration, and unset it when our VM is not protected.
> 
> Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> ---
> NOTES:
> * I wanted to have a discussion starter fast, there are multiple open
> questions.
> 
> * Doing more than one system_resets() is hackish.  We
> should look into this.
> 
> * The user interface implications of this patch are also an ugly can of
> worms. Needs to be discussed.
> 
> * We should consider keeping the original value if !pv and restoring it
> on pv --> !pv, instead of forcing to unset when leaving pv, and actually
> not forcing unset when !pv.
> 
> * It might make sense to do something like this in virtio core, but I
>   decided start the discussion with a ccw-only change.
> 
> * Maybe we need a machine option that enables this sort of behavior,
> especially if we decide not to do the conserving/restoring.
> 
> * Lightly tested.
> ---
>  hw/s390x/s390-virtio-ccw.c |  4 ++--
>  hw/s390x/virtio-ccw.c      | 13 +++++++++++++
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
> index 0f4455d1df..996124f152 100644
> --- a/hw/s390x/s390-virtio-ccw.c
> +++ b/hw/s390x/s390-virtio-ccw.c
> @@ -337,7 +337,7 @@ static void s390_machine_unprotect(S390CcwMachineState *ms)
>          ms->pv = false;
>      }
>      migrate_del_blocker(pv_mig_blocker);
> -    qemu_balloon_inhibit(false);
> +    subsystem_reset();
>  }
>  
>  static int s390_machine_protect(S390CcwMachineState *ms)
> @@ -346,7 +346,6 @@ static int s390_machine_protect(S390CcwMachineState *ms)
>      CPUState *t;
>      int rc;
>  
> -    qemu_balloon_inhibit(true);
>      if (!pv_mig_blocker) {
>          error_setg(&pv_mig_blocker,
>                     "protected VMs are currently not migrateable.");
> @@ -384,6 +383,7 @@ static int s390_machine_protect(S390CcwMachineState *ms)
>      if (rc) {
>          goto out_err;
>      }
> +    subsystem_reset();
>      return rc;
>  
>  out_err:
> diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> index 13f57e7b67..48bb54f68e 100644
> --- a/hw/s390x/virtio-ccw.c
> +++ b/hw/s390x/virtio-ccw.c
> @@ -869,12 +869,24 @@ static void virtio_ccw_notify(DeviceState *d, uint16_t vector)
>      }
>  }
>  
> +static inline void virtio_ccw_pv_enforce_features(VirtIODevice *vdev)
> +{
> +    S390CcwMachineState *ms = S390_CCW_MACHINE(qdev_get_machine());
> +
> +    if (ms->pv) {
> +        virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
> +    } else {
> +        virtio_clear_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
> +    }
> +}
> +
>  static void virtio_ccw_reset(DeviceState *d)
>  {
>      VirtioCcwDevice *dev = VIRTIO_CCW_DEVICE(d);
>      VirtIODevice *vdev = virtio_bus_get_device(&dev->bus);
>      VirtIOCCWDeviceClass *vdc = VIRTIO_CCW_DEVICE_GET_CLASS(dev);
>  
> +    virtio_ccw_pv_enforce_features(vdev);
>      virtio_ccw_reset_virtio(dev, vdev);
>      if (vdc->parent_reset) {
>          vdc->parent_reset(d);
> @@ -1103,6 +1115,7 @@ static void virtio_ccw_pre_plugged(DeviceState *d, Error **errp)
>      if (dev->max_rev >= 1) {
>          virtio_add_feature(&vdev->host_features, VIRTIO_F_VERSION_1);
>      }
> +    virtio_ccw_pv_enforce_features(vdev);
>  }
>  
>  /* This is called by virtio-bus just after the device is plugged. */
> 
> base-commit: 8665f2475f5999d4c9f33598f1360f0b0797c489
> 

I asked this question already to Michael (cc) via a different channel,
but hare is it again:

Why does the balloon driver not support VIRTIO_F_IOMMU_PLATFORM? It is
absolutely not clear to me. The introducing commit mentioned that it
"bypasses DMA". I fail to see that.

At least the communication via the SG mechanism should work perfectly
fine with an IOMMU enabled. So I assume it boils down to the pages that
we inflate/deflate not being referenced via IOVA?

I don't think they have to be IOVA addresses. We're neither reading nor
writing these pages. We really speak about "physical memory in the
system" when ballooning. Everything else doesn't really make sense.
There is no need to map/unmap pages we inflate/deflate AFAIKs.

IMHO, we should not try to piggy-back on VIRTIO_F_IOMMU_PLATFORM here,
but instead explicitly disable it either in the hypervisor or the guest.

I hope someone can clarify what the real issue with an IOMMU and
ballooning is, because I'll be having the same "issue" with virtio-mem.

-- 
Thanks,

David / dhildenb