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

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:

quoted

On 8 July 2015 at 17:37, Marc Zyngier [off-list ref] wrote:

quoted

On 08/07/15 17:06, Peter Maydell wrote:

quoted

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.

quoted

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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