Re: [PATCH v2 1/1] virtio-ccw: auto-manage VIRTIO_F_IOMMU_PLATFORM if PV

[Janosch Frank]

On 5/28/20 1:21 PM, Cornelia Huck wrote:
> On Fri, 22 May 2020 23:04:51 +0200
> Halil Pasic <pasic@linux.ibm.com> wrote:
> 
>> On Wed, 20 May 2020 12:23:24 -0400
>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>
>>> On Fri, May 15, 2020 at 12:11:55AM +0200, Halil Pasic wrote:  
>>>> 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 VMs, as the device can access only memory addresses that are
>>>> in pages that are currently shared (only the guest can share/unsare its
>>>> pages).
>>>>
>>>> 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. Since the correctness of the
>>>> paravirtualized virtio devices depends (and thus in a sense the
>>>> correctness of the hypervisor) it, then the hypervisor should have the
>>>> last word about whether VIRTIO_F_ACCESS_PLATFORM is to be presented or
>>>> not.  
>>>
>>> So, how about this: switch iommu to on/off/auto.  
>>
>> Many thanks for the reveiw, and sorry about the delay on my side. We
>> have holidays here in Germany and I was not motivated enough up until
>> now to check on my mails.
>>
>>
>> I've actually played  with the thought of switching iommu_platform to 
>> 'on/off/auto', but I didn't find an easy way to do it. I will look
>> again. This would be the first property of this kind in QEMU, or?
> 
> virtio-pci uses it for 'disable-legacy'.
> 
>>
>> The 'on/off/auto' would be certainly much cleaner form user-interface
>> perspective. The downsides are that it is more invasive, and more
>> complicated. I'm afraid that it would also leave more possibilities for
>> user error.
> 
> To me, on/off/auto sounds like a reasonable thing to do.
> 
> What possibilities of 'user error' do you see? Shouldn't we fence off
> misconfigurations, if the consequences would be disastrous?
> 
>>
>>>  Add a property with a
>>> reasonable name "allow protected"?  If set allow switch to protected
>>> memory and also set iommu auto to on by default.  If not set then don't.
>>>  
>>
>> I think we have "allow protected" already expressed via cpu models. I'm
>> also not sure how libvirt would react to the idea of a new machine
>> property for this. You did mean "allow protected" as machine property,
>> or?
> 
> "Unpack facility in cpu model" means "guest may transition into pv
> mode", right? What does it look like when the guest actually has
> transitioned?

Well, we don't sync the features that the protected guest has back into
QEMU. So basically the VM doesn't really change except for ms->pv now
being true.



> 
>>
>> AFAIU "allow protected" would be required for the !PV to PV switch, and
>> we would have to reject paravirtualized devices with iommu_platform='off'
>> on VM construction or hotplug (iommu_platform='auto/on' would be fine).
>>
>> Could you please confirm that I understood this correctly?
>>
>>
>>> This will come handy for other things like migrating to hosts without
>>> protected memory support.
>>>   
>>
>> This is already covered by cpu model AFAIK.
> 
> I don't think we'd want to migrate between pv and non-pv anyway?

What exactly do you mean by that?
I'd expect that the VM can either be migrated in PV or non-PV mode and
not in a transition phase.

> 
>>
>>>
>>> Also, virtio now calls this PLATFORM_ACCESS, maybe we should rename
>>> the property (keeping old one around for compat)?  
>>
>> You mean the like rename 'iommu_platform' to 'platform_access'? I like
>> the idea, but I'm not sure libvirt will like it as well. Boris any
>> opinions?
>>
>>> I feel this will address lots of complaints ...
>>>   
>>>> Currently presenting a PV guest with a (paravirtualized) virtio-ccw
>>>> device has catastrophic consequences for the VM (after the hypervisors
>>>> access to protected memory). This is especially grave in case of device
>>>> hotplug (because in this case the guest is more likely to be in the
>>>> middle of something important).
>>>>
>>>> Let us manage the VIRTIO_F_ACCESS_PLATFORM virtio feature automatically
>>>> for virtio-ccw devices, i.e. force it before we start the protected VM.
>>>> If the VM should cease to be protected, the original value is restored.
>>>>
>>>> Signed-off-by: Halil Pasic <pasic@linux.ibm.com>  
>>>
>>>
>>> I don't really understand things fully but it looks like you are
>>> changing features of a device.  If so this bothers me, resets
>>> happen at random times while driver is active, and we never
>>> expect features to change.
>>>  
>>
>> Changing the device features is IMHO all right because the features can
>> change only immediately after a system reset and before the first vCPU
>> is run. That is ensured by two facts.
>>
>>
>> First, the feature can only change when ms->pv changes. That is on the
>> first reset after the VM entered or left the "protected virtualization"
>> mode of operation. And that switch requires a system reset. Because the
>> PV switch is initiated by the guest, and the guest is rebooted as a
>> consequence, the guest will never observe the change in features.
> 
> This really needs more comments, as it is not obvious to the casual
> reader. (I also stumbled over the resets.)
> 
> But I wonder whether we are actually missing those subsystems resets
> today?
> 
>

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