Re: [Qemu-devel] [PATCH 0/2] Let boards state maximum RAM limits in QEMUMachine struct

[Anthony Liguori]

On 03/21/2011 12:47 PM, Peter Maydell wrote:
> This fairly simple patchset adds a new 'max_ram' field to the QEMUMachine
> structure so that a board model can specify the maximum RAM it will accept.
> We can then produce a friendly diagnostic message when the user tries to
> start qemu with a '-m' option asking for more RAM than that. (Currently
> most of the ARM devboard models respond with an obscure guest crash when
> the guest tries to access RAM and finds device registers instead.)
>
> If no maximum size is specified we default to the old behaviour of
> "do not impose any limit".
>
> The advantage of doing this in vl.c rather than in each board (apart
> from avoiding code duplication) is that we can distinguish between
> "the user asked for more RAM than we support" (an error) and "the global
> default RAM size is more than our maximum" (just cap the RAM size to
> the board maximum).
>
> I did think about also adding a "default_ram" field so each board could
> default to the same amount of RAM that real hardware has, but since we
> allocate RAM up front rather than only when accessed, this would push the
> initial default allocated RAM size up from 128MB to 1GB on models like
> the PBX A9, and it didn't seem very friendly to do that. I'm happy
> to be persuaded back again, though :-)

I don't have any objections to these patches, but it makes me wonder if 
we should try to go further.

There are certainly a lot of use-cases where the boards being emulated 
are extremely sensitive to the options specified.   I wonder if we ought 
to provide a better way for boards to participate in command line 
validation.  For instance, I know that many boards can only accept 
certain RAM sizes that map to realistic DIMM sizes.

I don't have any great ideas here but wonder if it's something we should 
more seriously consider.

Regards,

Anthony Liguori

> This patchset was prompted by bugs like:
> https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/584480
> https://bugs.launchpad.net/ubuntu/+source/rootstock/+bug/570588
>
> Peter Maydell (2):
>    Allow boards to specify maximum RAM size
>    hw: Add maximum RAM specifications for ARM devboard models
>
>   hw/boards.h       |    1 +
>   hw/integratorcp.c |    1 +
>   hw/realview.c     |   11 +++++++++++
>   hw/versatilepb.c  |    5 +++++
>   vl.c              |   16 +++++++++++++++-
>   5 files changed, 33 insertions(+), 1 deletions(-)