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

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-05-19 20:18, 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.
> 
> 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.

Jan


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