Re: [PATCH v3 0/9] Generalize memory encryption models

[Janosch Frank]

On 6/26/20 12:58 PM, Daniel P. Berrangé wrote:
> On Fri, Jun 26, 2020 at 11:29:03AM +0100, Dr. David Alan Gilbert wrote:
>> * Janosch Frank (frankja@linux.ibm.com) wrote:
>>> On 6/26/20 11:32 AM, Daniel P. Berrangé wrote:
>>>> On Fri, Jun 26, 2020 at 11:01:58AM +0200, Janosch Frank wrote:
>>>>> On 6/26/20 8:53 AM, David Hildenbrand wrote:
>>>>>>>>>> Does this have any implications when probing with the 'none' machine?
>>>>>>>>>
>>>>>>>>> I'm not sure.  In your case, I guess the cpu bit would still show up
>>>>>>>>> as before, so it would tell you base feature availability, but not
>>>>>>>>> whether you can use the new configuration option.
>>>>>>>>>
>>>>>>>>> Since the HTL option is generic, you could still set it on the "none"
>>>>>>>>> machine, though it wouldn't really have any effect.  That is, if you
>>>>>>>>> could create a suitable object to point it at, which would depend on
>>>>>>>>> ... details.
>>>>>>>>>
>>>>>>>>
>>>>>>>> The important point is that we never want the (expanded) host cpu model
>>>>>>>> look different when either specifying or not specifying the HTL
>>>>>>>> property.
>>>>>>>
>>>>>>> Ah, yes, I see your point.  So my current suggestion will satisfy
>>>>>>> that, basically it is:
>>>>>>>
>>>>>>> cpu has unpack (inc. by default) && htl specified
>>>>>>>         => works (allowing secure), as expected
>>>>>>
>>>>>> ack
>>>>>>
>>>>>>>
>>>>>>> !cpu has unpack && htl specified
>>>>>>>         => bails out with an error
>>>>>>
>>>>>> ack
>>>>>>
>>>>>>>
>>>>>>> !cpu has unpack && !htl specified
>>>>>>>         => works for a non-secure guest, as expected
>>>>>>>         => guest will fail if it attempts to go secure
>>>>>>
>>>>>> ack, behavior just like running on older hw without unpack
>>>>>>
>>>>>>>
>>>>>>> cpu has unpack && !htl specified
>>>>>>>         => works as expected for a non-secure guest (unpack feature is
>>>>>>>            present, but unused)
>>>>>>>         => secure guest may work "by accident", but only if all virtio
>>>>>>>            properties have the right values, which is the user's
>>>>>>>            problem
>>>>>>>
>>>>>>> That last case is kinda ugly, but I think it's tolerable.
>>>>>>
>>>>>> Right, we must not affect non-secure guests, and existing secure setups
>>>>>> (e.g., older qemu machines). Will have to think about this some more,
>>>>>> but does not sound too crazy.
>>>>>
>>>>> I severely dislike having to specify things to make PV work.
>>>>> The IOMMU is already a thorn in our side and we're working on making the
>>>>> whole ordeal completely transparent so the only requirement to make this
>>>>> work is the right machine, kernel, qemu and kernel cmd line option
>>>>> "prot_virt=1". That's why we do the reboot into PV mode in the first 
>>>>> place.
>>>>>
>>>>> I.e. the goal is that if customers convert compatible guests into
>>>>> protected ones and start them up on a z15 on a distro with PV support
>>>>> they can just use the guest without having to change XML or command line
>>>>> parameters.
>>>>
>>>> If you're exposing new features to the guest machine, then it is usually
>>>> to be expected that XML and QEMU command line will change. Some simple
>>>> things might be hidable behind a new QEMU machine type or CPU model, but
>>>> there's a limit to how much should be hidden that way while staying sane.
>>>>
>>>> I'd really expect the configuration to change when switching a guest to
>>>> a new hardware platform and wanting major new functionality to be enabled.
>>>> The XML / QEMU config is a low level instantiation of a particular feature
>>>> set, optimized for a specific machine, rather than a high level description
>>>> of ideal "best" config independent of host machine.
>>>
>>> You still have to set the host command line and make sure that unpack is
>>> available. Currently you also have to specify the IOMMU which we like to
>>> drop as a requirement. Everything else is dependent on runtime
>>> information which tells us if we need to take a PV or non-PV branch.
>>> Having the unpack facility should be enough to use the unpack facility.
>>>
>>> Keep in mind that we have no real concept of a special protected VM to
>>> begin with. If the VM never boots into a protected kernel it will never
>>> be protected. On a reboot it drops from protected into unprotected mode
>>> to execute the bios and boot loader and then may or may not move back
>>> into a protected state.
>>
>> My worry isn't actually how painful adding all the iommu glue is, but
>> what happens when users forget; especially if they forget for one
>> device.
>>
>> I could appreciate having a machine option to cause iommu to then get
>> turned on with all other devices; but I think also we could do with
>> something that failed with a nice error if an iommu flag was missing.
>> For SEV this could be done pretty early, but for power/s390 I guess
>> you'd have to do this when someone tried to enable secure mode, but
>> I'm not sure you can tell.
> 
> What is the cost / downside of turning on the iommu option for virtio
> devices ? Is it something that is reasonable for a mgmt app todo
> unconditionally, regardless of whether memory encryption is in use,
> or will that have a negative impact on things ?

speed, memory usage and compatibility problems.
There might also be a problem with s390 having to use <=2GB iommu areas
in the guest, I need to check with Halil if this is still true.

Also, if the default or specified IOMMU buffer size isn't big enough for
your IO workload the guest is gonna have a very bad time. I.e. if
somebody has an alternative implementation of bounce buffers we'd be
happy to take it :)

> 
> Regards,
> Daniel
>

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