[RFC PATCH] KVM: arm/arm64: Don't let userspace update CNTVOFF once guest is running

[Jan Kiszka]

On 2015-07-09 12:22, Christoffer Dall wrote:
> Hi Peter and Marc,
> 
> [cc'ing Paolo for his input on x86 timekeeping]
> 
> On Wed, Jul 08, 2015 at 08:13:59PM +0100, Peter Maydell wrote:
>> On 8 July 2015 at 17:37, Marc Zyngier <marc.zyngier at arm.com> wrote:
>>> On 08/07/15 17:06, Peter Maydell wrote:
>>>> I'd prefer it if somebody could investigate to see why QEMU
>>>> is actually doing this -- so far we just have speculation.
>>>
>>> I'd prefer that too, but so far people seem to be more comfortable
>>> waiting for the issue to fix itself. In the meantime, VMs are broken in
>>> weird and wonderful ways, and I don't think the current status-quo helps
>>> anyone.
>>
>> Putting in a patch which might not be the right fix isn't
>> necessarily a good plan either...
>>
>> Does has_run_once get cleared if we do a re-VCPU_INIT
>> of a CPU that's run before? (We need to allow rewriting
>> of guest state at that point so that "reset VM and
>> load migration state" behaves correctly.)
> 
> no, it does not, has_run_once is set the first time a VCPU is run and is
> currently *never* cleared.
> 
>>
>> I suspect Jan is right and we really need to distinguish
>> the KVM_PUT_*_STATE levels in ARM QEMU. This probably
>> implies some kind of whitelist/override mechanism, since
>> by and large we neither know nor want to know the
>> semantics for system registers, we leave that up to the
>> kernel.
>>
>> Q: if you have a running VM, and you pause it for
>> an hour, what should the CNTVCT register do? Presumably
>> it should not advance, but how do we arrange for that
>> to happen?
>>
> 
> I think the CNTVCT should not advance when the VM is not scheduled, so
> if we pause the VM or starve all the VCPUs for enough time, the guest
> should not see time progressing, since otherwise the guest scheduler
> cannot maintain fairness and you're bound to see spurious RCU stalls
> etc.
> 
> That's exactly why a guest can read both a virtual and physical counter
> and it is an area where you simply want some level of
> paravirtualization.  I haven't studied how/if Linux deals with this at
> all.
> 
> So I think adjusting CNTVOFF should be managed by the kernel for the
> pause/starvation scenario (which I think Avi once told me x86 does too -
> does anyone know the current state of the art?).
> 
> So the only situation where I think userspace should adjust the CNTVOFF
> value is for migration where we are talking about a brand new VM with
> has_run_once clear.
> 
> Thus, if we were designing this from scratch now, the API should
> be to return an error when trying to set KVM_REG_ARM_TIMER_CNT after the
> VM has run once, but it's too late for that as we would break userspace.
> The best alternative IMHO would be to merge Marc's patch and fix CNTVOFF
> in the kernel side as well, and finally also fix QEMU so that it doesn't
> try to do the thing that future kernels will ignore.

Fixing QEMU to only write on KVM_PUT_FULL_STATE - yes, that should be
done, but I don't think the approach for the kernel is generally right.
The kernel should not do any policing on user space requests to change
the VCPU or VM state unless

 - security is affected
 - userspace lacks information, thus cannot decide correctly
 - legacy userspace has a bug, we can detect it and want to fix that up
   without affecting future userspace that has a reason to do it
   differently

Regarding CNTVOFF, the first two criteria do not apply for sure. Maybe
the last one, don't know. Just think of the hypothetical scenario that a
userspace VM debugger wants to inject certain register manipulations. If
you block this by some hidden VM state like proposed, that feature would
no longer be implementable easily.

Jan


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