Re: [Qemu-devel] [PATCH] kvm: align ram_size to page boundary

[Avi Kivity <avi@redhat.com>]

On 06/17/2012 03:43 PM, Blue Swirl wrote:
> On Sun, Jun 17, 2012 at 11:51 AM, Avi Kivity <avi@redhat.com> wrote:
>> On 06/17/2012 02:47 PM, Jan Kiszka wrote:
>>>>>
>>>>> I think this should rather go into generic code.
>>>>
>>>> To be honest, I put this in kvm-specific code because vl.c doesn't have
>>>> TARGET_PAGE_ALIGN.  Maybe we should have machine->page_size or
>>>> machine->ram_alignment.
>>>>
>>>>> What sense does it make
>>>>> to have partial pages with TCG?
>>>>
>>>> Why impose an artificial restriction?
>>>
>>> Beca...
>>>
>>>>
>>>> (answer: to reduce differences among various accelerators)
>>>>
>>>
>>> Oh, you found the answer. :)
>>
>> Reducing round-trips across the Internet.
>>
>>>
>>> At least, it should be enforce for the x86 target, independent of the
>>> accelerator.
>>
>> Yeah.  So there's machine->page_size or machine->ram_alignment.  Not
>> sure which is best.
> 
> The boards should make sure that the amount of RAM is feasible with
> the board memory slots. It's not possible to put 256kb SIMMs to a slot
> that expects 1GB DIMMs. We can allow some flexibility there though,
> I'm not sure if the current chipsets would support very much memory if
> we followed the docs to the letter.

Right. And generally memory modules are sized a power of two, creating
the silly "mega == 1048576" movement.

> 
> Maybe strtosz() should just enforce 1MB granularity.

strtosz() is much too general.  We could do it in vl.c without trouble.
 However, it takes away our ability to emulate a "640k should be enough
for everyone" machine.

> 
> What about ballooning (memory hotplug?), can that reduce the memory by
> smaller amount than page size?

Ballooning removes individual pages, that has no effect on the slot size.

-- 
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