Re: [Qemu-devel] [PATCH 4/7 v2] KVM regsync: Add register bitmap parameter to do_kvm_cpu_synchronize_state

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 16/01/13 17:05, Marcelo Tosatti wrote:

> The S/390 problem, from
> http://lists.nongnu.org/archive/html/qemu-devel/2012-11/msg02213.html:
> 
> ">>> The kvm register sync needs to happen in the kvm register sync
>>>> function :)
>>> That would eliminate the whole purpose of sync regs and forces us to
>>> have an
>>> expensive ioctl on lots of exits (again). I would prefer to sync the 
>>> registers
>>> that we never need in qemu just here.
>>
>> That's why the register sync has different stages.
> 
> Not the get_register. Which is called on every synchronize_state. Which
> happen 
> quite often
> on s390."
> 
> But wait: on these S/390 codepaths, you do GET_REGS already, via
> cpu_synchronize_state.
> 
> So on S/390
> 
> - cpu_synchronize_state(env)
> - read any register from env
> 
> Is not valid? This is what generic code assumes.

TO recap the motiviation:

cpu_synchronize_state on s390 currently updates any register in env that is
used by qemu (general purpose, prefix, psw, control and access) in the normal
runtime. it turns out we have all of these regs in kvm_run, so we can do 
synchronize states without doing an additional ioctl call.
Now, for life migration and dump we need some additional registers (which are
only accessable via onereg interface). So synchronize_state would need to
do 3 or 4 additional system calls on the hot path, only to take care of 
something that is not on the hot path at all.
For historic reasons, we have one exit code for almost all exits. Therefore,
we need to call synchronize_states almost always.
We could now start to have a poor mans synchronize_state in arch code, but
that would collide with common code synchronize_state if done at the wrong
time. Thus we want to make common code capable of having only a subset of
the register synched - by making it possible to sync the other regs later
on if needed without wiping the former sync.

Makes sense?

Christian