Re: [Qemu-devel] [RFC] Memory API

[Anthony Liguori]

On 05/19/2011 01:50 PM, Jan Kiszka wrote:
> On 2011-05-19 20:18, Anthony Liguori wrote:
> > Well, not really.
> >
> > kvm.ko has a global mapping of RAM regions and currently only allows
> > code execution from RAM.
> >
> > This means the only way for QEMU to enable SMM support is to program the
> > global RAM regions table to enable allow RAM access for the VGA region.
> >
> > The problem with this is that it's perfectly conceivable to have CPU 0
> > in SMM mode while CPU 1 is doing MMIO to the VGA planar.
> >
> > The same problem exists with PAM.  It would be much easier to implement
> > PAM correctly in QEMU if it were possible to execute code via MMIO as we
> > could just mark the BIOS memory as non-RAM and deal with the dispatch
> > ourselves.
>
> If we already have to change KVM (I guess we have to), let's better add
> per-CPU memory slot support. That will allow to switch between VGA and
> SMRAM without costly dispatching. At this chance, I think we also need
> some support for half-MMIO (MMIO on write, RAM on read) for proper flash
> support.

This is needed for PAM too.

But RAM isn't mapped per-CPU so this is at best an optimization.  You 
can (and do) execute instructions out of non-RAM memory though.  I think 
if we lifted this restriction in KVM, it would allow us to handle 
SMRAM/PAM in a more thorough way.

Regards,

Anthony Liguori

> Jan