Re: xilinx-zynq-a9: cannot set up guest memory 'zynq.ext_ram'

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 20 Aug 2021 15:39:27 +0100
Peter Maydell <peter.maydell@linaro.org> wrote:

> On Fri, 20 Aug 2021 at 15:34, David Hildenbrand <david@redhat.com> wrote:
> >
> > On 20.08.21 16:22, Bin Meng wrote:  
> > > Hi Philippe,
> > >
> > > On Fri, Aug 20, 2021 at 10:10 PM Philippe Mathieu-Daudé
> > > <philmd@redhat.com> wrote:  
> > >>
> > >> Hi Bin,
> > >>
> > >> On 8/20/21 4:04 PM, Bin Meng wrote:  
> > >>> Hi,
> > >>>
> > >>> The following command used to work on QEMU 4.2.0, but is now broken
> > >>> with QEMU head.
> > >>>
> > >>> $ qemu-system-arm -M xilinx-zynq-a9 -display none -m 40000000
> > >>> -nographic -serial /dev/null -serial mon:stdio -monitor null -device
> > >>> loader,file=u-boot-dtb.bin,addr=0x4000000,cpu-num=0
> > >>> qemu-system-arm: cannot set up guest memory 'zynq.ext_ram': Cannot
> > >>> allocate memory  
> 
> > -m 40000000
> >
> > corresponds to 38 TB if I am not wrong. Is that really what you want?  
> 
> Probably not, because the zynq board's init function does:
> 
>     if (machine->ram_size > 2 * GiB) {
>         error_report("RAM size more than 2 GiB is not supported");
>         exit(EXIT_FAILURE);
>     }
> 
> It seems a bit daft that we allocate the memory before we do
> the size check. This didn't use to be this way around...
> 
> Anyway, I think the cause of this change is commit c9800965c1be6c39
> from Igor. We used to silently cap the RAM size to 2GB; now we
> complain. Or at least we would complain if we hadn't already
> tried to allocate the memory and fallen over...

That's because RAM (as host resource) is now separated
from device model (machine limits) and is allocated as
part of memory backend initialization (in this case
'create_default_memdev') before machine_run_board_init()
is run.

Maybe we can consolidate max limit checks in
create_default_memdev() by adding MachineClass::max_ram_size
but that can work only in default usecase (only '-m' is used).

However if user creates backend explicitly, there aren't any
clue about machine limits. We basically don't know what
backend is created for at the time it's initialized
(which includes RAM allocation, it might be created for VM's
RAM or ram/storage for some other device).

> 
> -- PMM
>