Daniel P. Berrange wrote:
> On Fri, Feb 01, 2008 at 11:53:02AM -0600, Anthony Liguori wrote:
>
>> Ian Jackson wrote:
>>
>>> Anthony Liguori writes ("[Qemu-devel] Re: [kvm-devel] [PATCH 1/6] Use
>>> correct types to enable > 2G support"):
>>>
>>>
>>>> The alternative is to change all the places that assume phys_ram_base +
>>>> PA which I don't like very much.
>>>>
>>>>
>>> We would ideally like to do this for Xen, at least in the places we
>>> care about. (Xen uses less of the qemu tree than KVM, I think.)
>>>
>>>
>> Support for the map cache in the Xen tree is a rather big change that
>> I'm not going to attempt to support it in this patch series.
>>
>> I'd rather preserve the phys_ram_base + PA assumption because it allows
>> us to be able to do support > 1 page DMA operations for our virtual IO
>> drivers. If you break the assumption that physically contiguous memory
>> in the guest is virtual contiguous memory in the host, things get pretty
>> ugly.
>>
>
> Well Xen i386 has no choice but to use the map cache, since PAE lets
> i386 guests have as much as 100 GB of memory & there's no way you can
> map that into QEMU's 32-bit userspace. So if virt IO has a dependancy
> on contigious memory access in QEMU its not going to play nice with
> Xen.
>
For KVM (and it sounds like QEMU), we're just making the statement that
32-bit hosts cannot support > 2GB guests. I know that's a regression
for Xen but in all fairness, I did raise this as an objection when the
map cache was first introduced :-)
virtio could still be made to work with map cache. You would just have
to change it to be able to map more than one page contiguously. As I
mentioned though, it just starts getting ugly.
Regards,
Anthony Liguori
> Dan.
>
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