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

[Avi Kivity <avi@redhat.com>]

On 05/19/2011 09:18 PM, Anthony Liguori wrote:
> 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.

kvm needs updates to support SMM; I already outlined them several months 
ago.

>
> The same problem exists with PAM. 

PAM is a completely different problem.  The changes are global and fit 
kvm slot management.

> 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.

You need to emulate everything.  We're probably not far from that.  
However there may be a significant performance loss.


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