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

[Ani Sinha]

On Tue, Sep 19, 2023 at 1:13 PM David Hildenbrand <david@redhat.com> wrote:
>
> 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).
>

Ok fair enough. The existing scheme clearly does not support 32-bit
memory hotplug.

We do have a slight improvement with our original test case. I will
send an updated patch that passed all unit tests :

$ ./qemu-system-x86_64 -machine pc-i440fx-8.2 -cpu pentium -m
size=10G, -monitor stdio -qmp tcp:0:5555,server,nowait
QEMU 8.1.50 monitor - type 'help' for more information
qemu-system-x86_64: Address space limit 0xffffffff < 0x2bfffffff
phys-bits too low (32)

[anisinha@rhel9-box build]$ ./qemu-system-x86_64 -machine
pc-i440fx-8.1 -cpu pentium -m size=10G, -monitor stdio -qmp
tcp:0:5555,server,nowait
QEMU 8.1.50 monitor - type 'help' for more information
VNC server running on ::1:5900
(qemu)