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

[Avi Kivity <avi@redhat.com>]

On 12/22/2011 02:49 PM, Christian Borntraeger wrote:
> > 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. 

Those are only memory accesses coming from the cpu, yes?  Why does
userspace have to access them at all?  I imagine DMA ignores it
completely since it doesn't come from the cpu.

> 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.

I doubt other archs will benefit, but it makes sense to reserve the
space generically as you did.

-- 
error compiling committee.c: too many arguments to function