Re: [PATCH] mem/x86: add processor address space check for VM memory

[David Hildenbrand <david@redhat.com>]

On 08.09.23 16:12, Ani Sinha wrote:
> 
> 
>> On 08-Sep-2023, at 3:58 PM, David Hildenbrand <david@redhat.com> wrote:
>>
>> On 08.09.23 11:50, Ani Sinha wrote:
>>> Depending on the number of available address bits of the current processor, a
>>> VM can only use a certain maximum amount of memory and no more. This change
>>> makes sure that a VM is not configured to have more memory than what it can use
>>> with the current processor settings when started. Additionally, the change adds
>>> checks during memory hotplug to ensure that the VM does not end up getting more
>>> memory than what it can actually use after hotplug.
>>> Currently, both the above checks are only for pc (x86) platform.
>>> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1235403
>>> CC: imammedo@redhat.com
>>> Signed-off-by: Ani Sinha <anisinha@redhat.com>
>>> ---
>>>   hw/i386/pc.c           | 45 ++++++++++++++++++++++++++++++++++++++++++
>>>   hw/mem/memory-device.c |  6 ++++++
>>>   include/hw/boards.h    |  9 +++++++++
>>>   3 files changed, 60 insertions(+)
>>> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
>>> index 54838c0c41..f84e4c4916 100644
>>> --- a/hw/i386/pc.c
>>> +++ b/hw/i386/pc.c
>>> @@ -31,6 +31,7 @@
>>>   #include "hw/i386/topology.h"
>>>   #include "hw/i386/fw_cfg.h"
>>>   #include "hw/i386/vmport.h"
>>> +#include "hw/mem/memory-device.h"
>>>   #include "sysemu/cpus.h"
>>>   #include "hw/block/fdc.h"
>>>   #include "hw/ide/internal.h"
>>> @@ -1006,6 +1007,17 @@ void pc_memory_init(PCMachineState *pcms,
>>>           exit(EXIT_FAILURE);
>>>       }
>>>   +    /*
>>> +     * check if the VM started with more ram configured than max physical
>>> +     * address available with the current processor.
>>> +     */
>>> +    if (machine->ram_size > maxphysaddr + 1) {
>>> +        error_report("Address space limit 0x%"PRIx64" < 0x%"PRIx64
>>> +                     " (max configured memory), phys-bits too low (%u)",
>>> +                     maxphysaddr, machine->ram_size, cpu->phys_bits);
>>> +        exit(EXIT_FAILURE);
>>> +    }
>>
>> ... I know that this used to be a problem in the past, but nowadays we already do have similar checks in place?
>>
>> $ ./build/qemu-system-x86_64 -m 4T -machine q35,memory-backend=mem0 -object memory-backend-ram,id=mem0,size=4T,reserve=off
>> qemu-system-x86_64: Address space limit 0xffffffffff < 0x5077fffffff phys-bits too low (40)
> 
> So you are saying that this is OK and should be allowed? On a 32 bit processor that can access only 4G memory, I am spinning up a 10G VM.

Would that 32bit process have PAE (Physical Address Extension) and still 
be able to access that memory?

-- 
Cheers,

David / dhildenb