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

[Avi Kivity]

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?

For extra points run the translator in guest context.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.