Re: [PATCH/PFC 0/2] s390 host support

[Carsten Otte <cotte-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>]

Avi Kivity wrote:
> We'll want to keep a vcpu fd.  If the vcpu is idle we'll be asleep in 
> poll() or the like, and we need some kind of wakeup mechanism.
Our userspace does idle/wakeup differently:
One cpu exits sys_s390host_sie, and the intercept code indicates a 
halt with interrupts enabled (cpu idle loop). Now userland parks our 
vcpu thread in pthread_cond_wait. Once we want to wakeup this thread, 
either by interprocessor signal (need_resched and such) or due to an 
IO interrupt, we do a pthread_cond_signal to wakeup the thread again. 
The thread will now enter sys_s390host_sie, and after entering the 
vcpu context will execute the interrupt handler first.
The advantage of waiting in userland I see, is that userspace can dump 
interrupts to idle CPUs without kernel intervention. On the other 
hand, my brain hurts when thinking about userland passing vcpu fds to 
other threads/processes and when thinking about sys_fork().
In the end, you do the decision and we'll follow the way you lead to.

> I guess some of the difference stems from the fact that on x86, the 
> Linux pagetables are actually the hardware pagetables.  VT and SVM use a 
> separate page table for the guest which cannot be shared with the host. 
> This means that
> 
> - we need to teach the Linux mm to look at shadow page tables when 
> transferring dirty bits
> - when Linux wants to write protect a page, it has to modify the shadow 
> page tables too (and flush the guest tlbs, which is again a bit different)
> - this means rmap has to be extended to include kvm
> 
> I think that non-x86 have purely software page tables, maybe this make 
> things easier.
We do use hardware page tables too. Our hardware does know about 
mutiple levels of page translation, and does its part of maintaining 
different sets of dirty/reference bits for guest and host while 
running in the virtual machine context. This process is transparent 
for both virtual machine and host.
For the x86 part, I will spend some time to read the kvm code a little 
more.

so long,
Carsten

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