Re: [patch 0/3] RFC: provide synchronous registers in kvm_run

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

On 22/12/11 13:35, Avi Kivity wrote:
> On 12/22/2011 01:56 PM, Christian Borntraeger wrote:
>> Avi, Marcelo,
>>
>> currently userspace can access guest registers via several ioctls. Some
>> of these registers might be useful very often. Here the system call overhead
>> for ioctl can make an exit more expensive than necessary.
>> In a discussion with Alex Graf we concluded that it might be beneficial to
>> have a subset of registers available in kvm_run. (The ioctls will also be
>> available).
>>
>> This series provides a prototype implementation together with two example
>> users for s390.
>>
>>
> 
> Interesting.  Other archs emulate everything to do with registers in the
> kernel, so this is not a fast path.
> 
> What workload does this benefit?

My main concern was the prefix register (this is a per cpu register that
defines the address of two pages that are swapped with the pages at 0 for this cpu).
SMP on s390 is done that way (e.g. interrupt things are stored in page 0 for this cpu)
The storage that qemu sees is storage without prefix. For architecture compliance
we actually must check _every_ memory access if it hits the prefix/swpa area and 
the add/subtract the prefix value. 

I just added the ability to share other registers later after some discussions
with Alexander Graf because it seems to be doable for almost no cost.


Christian