Re: [Qemu-devel] Re: [PATCH 1/6] Use correct types to enable > 2G support

[Robert William Fuller <hydrologiccycle@gmail.com>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>> Fabrice Bellard wrote:
>>> Anthony Liguori wrote:
>>>> +    /* above 4giga memory allocation */
>>>> +    if (above_4g_mem_size > 0) {
>>>> +        ram_addr = qemu_ram_alloc(above_4g_mem_size);
>>>> +        cpu_register_physical_memory(0x100000000, 
>>>> above_4g_mem_size, ram_addr);
>>>> +    }
>>>> +
>>>
>>> Why do you need this ? All the RAM can be registered with a single 
>>> call.  I fear you need to do that because of KVM RAM handling 
>>> limitations.
>>
>> On the x86, there is a rather large hole at the top of memory.  
>> Currently, we do separate allocations around this whole.    You can't 
>> get away from doing multiple cpu_register_physical_memory calls here.  
>> We've discussed just allocating a single chunk with qemu_ram_alloc 
>> since so many places in QEMU assume that you can do phys_ram_base + PA.
>>
>> I think I'll change this too into a single qemu_ram_alloc.  That will 
>> fix the bug with KVM when using -kernel and large memory anyway :-)
> 
> Won't that cause all of the memory in the hole to be wasted?
> 
> You could munmap() it, but it's hardly elegant.
> 

Linux doesn't commit mapped memory until it's faulted.  As for other 
platforms, who knows?