> On 08-Sep-2023, at 7:46 PM, David Hildenbrand <da...@redhat.com> wrote:
>
> On 08.09.23 16:12, Ani Sinha wrote:
>>> On 08-Sep-2023, at 3:58 PM, David Hildenbrand <da...@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: imamm...@redhat.com
>>>> Signed-off-by: Ani Sinha <anisi...@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?
You are sidestepping my point. Sure, we can improve the condition check by
checking for PAE CPUID etc but that is not the issue I am trying too point out.
What if the processor did not have PAE? Would we allow a VM to have memory size
which the processor can’t access? There is no such check today it would seem.
