Re: [Qemu-devel] [RFC] Host vs Guest memory allocation

[Alexander Graf]

On 12.04.2010, at 17:09, Avi Kivity wrote:

> On 04/12/2010 05:55 PM, Richard Henderson wrote:
>> 
>>> You could reduce the overhead somewhat by using kvm for memory
>>> translation on hosts that support it. Of course tcg translation and
>>> syscall costs will grow by the exit overhead.
>> 
>> I've thought about this a bit, and what seemed to be the stickler is
>> what is the environment that runs in the guest?  TCG generated code
>> is of course fine, but what about the helper functions?  How can we
>> tell whether a given helper function can run in the restricted environment 
>> of the guest or whether it needs to transition back to the environment of 
>> the host to do its work?
> 
> I'd guess all helpers can run in guest context except those that cause a 
> transition to target kernel mode.
> 
>> I suppose the obvious solution is some sort of flag on the function that 
>> well-maintained ports will set.  But the whole marshalling thing is still 
>> pretty tricky.
> 
> Pass everything through memory; will there be many transitions apart from 
> trapping instructions and missing translations?

I don't see how that would help with the 64-on-32 issue. You still don't get a 
64 bit address space from running inside KVM.

Alex