Re: [PATCH] hw/i386/pc: fix max_used_gpa for 32-bit systems

[David Hildenbrand <david@redhat.com>]

On 19.09.23 08:18, Ani Sinha wrote:
> On Tue, Sep 19, 2023 at 9:53 AM Ani Sinha <anisinha@redhat.com> wrote:
>>
>> On Tue, Sep 19, 2023 at 9:20 AM Ani Sinha <anisinha@redhat.com> wrote:
>>>
>>> On Mon, Sep 18, 2023 at 9:28 PM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 18.09.23 17:56, Ani Sinha wrote:
>>>>> On Mon, Sep 18, 2023 at 8:59 PM David Hildenbrand <david@redhat.com> wrote:
>>>>>>
>>>>>> On 18.09.23 17:22, Ani Sinha wrote:
>>>>>>> On Mon, Sep 18, 2023 at 7:25 PM Ani Sinha <anisinha@redhat.com> wrote:
>>>>>>>>
>>>>>>>> 32-bit systems do not have a reserved memory for hole64 but they may have a
>>>>>>>> reserved memory space for memory hotplug. Since, hole64 starts after the
>>>>>>>> reserved hotplug memory, the unaligned hole64 start address gives us the
>>>>>>>> end address for this memory hotplug region that the processor may use.
>>>>>>>> Fix this. This ensures that the physical address space bound checking works
>>>>>>>> correctly for 32-bit systems as well.
>>>>>>>
>>>>>>> This patch breaks some unit tests. I am not sure why it did not catch
>>>>>>> it when I tested it before sending.
>>>>>>> Will have to resend after fixing the tests.
>>>>>>
>>>>>> Probably because they supply more memory than the system can actually
>>>>>> handle? (e.g., -m 4g on 32bit)?
>>>>>
>>>>> cxl tests are failing for example.
>>>>>
>>>>> $ ./qemu-system-i386 -display none -machine q35,cxl=on
>>>>> qemu-system-i386: Address space limit 0xffffffff < 0x1000fffff
>>>>> phys-bits too low (32)
>>>
>>> also another thing is:
>>>
>>> ./qemu-system-i386 -machine pc -m 128
>>> works but ...
>>>
>>> $ ./qemu-system-i386 -machine pc -m 128,slots=3,maxmem=1G
>>> qemu-system-i386: Address space limit 0xffffffff < 0x1f7ffffff
>>> phys-bits too low (32)
>>>
>>> or
>>>
>>> $ ./qemu-system-i386 -machine pc-i440fx-8.2 -accel kvm -m 128,slots=3,maxmem=1G
>>> qemu-system-i386: Address space limit 0xffffffff < 0x1f7ffffff
>>> phys-bits too low (32)
>>>
>>> but of course after the compat knob older pc machines work fine using
>>> the old logic :
>>>
>>> $ ./qemu-system-i386 -machine pc-i440fx-8.1 -accel kvm -m 128,slots=3,maxmem=1G
>>> VNC server running on ::1:5900
>>> ^Cqemu-system-i386: terminating on signal 2
>>
>> I dpn't know if we always need to do this but this code adds 1 GiB per
>> slot for device memory :
>>
>>      if (pcmc->enforce_aligned_dimm) {
>>           /* size device region assuming 1G page max alignment per slot */
>>           size += (1 * GiB) * machine->ram_slots;
>>       }
>>
>> For a 32-bit machine that is a lot of memory consumed in just alignment.
> 
> Let's look at an example when we get rid of all alignment stuff.
> 
> $ ./qemu-system-i386 -machine pc-i440fx-8.2 -m 512M,slots=1,maxmem=1G
> above 4G start: 0x100000000,above 4G size: 0x0
> qemu-system-i386: Address space limit 0xffffffff < 0x11ffffffe
> phys-bits too low (32)
> 
> So basically, above_4g_start = 4GiB. size = 0.
> Then it is adding the device memory which is 1GiB - 0.5 GiB = 0.5 GiB.
> So the  0x11ffffffe is exactly 4.5 GiB.
> 
> Anything above 4 GiB is beyond 32 bits.
> 

It's not worth worrying about memory devices for 32bit at all. For 
example Linux doesn't support memory hotplug on any 32bit system (not 
even with PAE and friends).

-- 
Cheers,

David / dhildenb