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

[Anthony Liguori]

On 05/19/2011 09:11 AM, Avi Kivity wrote:
> On 05/19/2011 05:04 PM, Anthony Liguori wrote:
> > Right, the chipset register is mainly used to program the contents of
> > SMM.
> >
> > There is a single access pin that has effectively the same semantics
> > as setting the chipset register.
> >
> > It's not a per-CPU setting--that's the point. You can't have one CPU
> > reading SMM memory at the exactly same time as accessing VGA.
> >
> > But I guess you can never have two simultaneous accesses anyway so
> > perhaps it's splitting hairs :-)
>
> Exactly - it just works.

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.

Would it be fundamentally hard to support this in KVM?  I guess you 
would need to put the VCPU in single step mode and maintain a page to 
copy the results into.

Regards,

Anthony Liguori

> btw, a way to implement it would be to have two memory maps, one for SMM
> and one for non-SMM, and select between them based on the CPU mode.