On Fri, 22 Sept, 2023, 10:25 pm Philippe Mathieu-Daudé, <phi...@linaro.org>
wrote:
> On 22/9/23 18:04, Ani Sinha wrote:
> > 32-bit x86 systems do not have a reserved memory for hole64. On those
> 32-bit
> > systems without PSE36 or PAE CPU features, hotplugging memory devices
> are not
> > supported by QEMU as QEMU always places hotplugged memory above 4 GiB
> boundary
> > which is beyond the physical address space of the processor. Linux
> guests also
> > does not support memory hotplug on those systems. Please see Linux
> > kernel commit b59d02ed08690 ("mm/memory_hotplug: disable the
> functionality
> > for 32b") for more details.
> >
> > Therefore, the maximum limit of the guest physical address in the
> absence of
> > additional memory devices effectively coincides with the end of
> > "above 4G memory space" region for 32-bit x86 without PAE/PSE36. When
> users
> > configure additional memory devices, after properly accounting for the
> > additional device memory region to find the maximum value of the guest
> > physical address, the address will be outside the range of the
> processor's
> > physical address space.
> >
> > This change adds improvements to take above into consideration.
> >
> > For example, previously this was allowed:
> >
> > $ ./qemu-system-x86_64 -cpu pentium -m size=10G
> >
> > With this change now it is no longer allowed:
> >
> > $ ./qemu-system-x86_64 -cpu pentium -m size=10G
> > qemu-system-x86_64: Address space limit 0xffffffff < 0x2bfffffff
> phys-bits too low (32)
> >
> > However, the following are allowed since on both cases physical address
> > space of the processor is 36 bits:
> >
> > $ ./qemu-system-x86_64 -cpu pentium2 -m size=10G
> > $ ./qemu-system-x86_64 -cpu pentium,pse36=on -m size=10G
> >
> > For 32-bit, without PAE/PSE36, hotplugging additional memory is no
> longer allowed.
> >
> > $ ./qemu-system-i386 -m size=1G,maxmem=3G,slots=2
> > qemu-system-i386: Address space limit 0xffffffff < 0x1ffffffff phys-bits
> too low (32)
> > $ ./qemu-system-i386 -machine q35 -m size=1G,maxmem=3G,slots=2
> > qemu-system-i386: Address space limit 0xffffffff < 0x1ffffffff phys-bits
> too low (32)
> >
> > A new compatibility flag is introduced to make sure pc_max_used_gpa()
> keeps
> > returning the old value for machines 8.1 and older.
> > Therefore, the above is still allowed for older machine types in order
> to support
> > compatibility. Hence, the following still works:
> >
> > $ ./qemu-system-i386 -machine pc-i440fx-8.1 -m size=1G,maxmem=3G,slots=2
> > $ ./qemu-system-i386 -machine pc-q35-8.1 -m size=1G,maxmem=3G,slots=2
> >
> > Further, following is also allowed as with PSE36, the processor has
> 36-bit
> > address space:
> >
> > $ ./qemu-system-i386 -cpu 486,pse36=on -m size=1G,maxmem=3G,slots=2
> >
> > After calling CPUID with EAX=0x80000001, all AMD64 compliant processors
> > have the longmode-capable-bit turned on in the extended feature flags
> (bit 29)
> > in EDX. The absence of CPUID longmode can be used to differentiate
> between
> > 32-bit and 64-bit processors and is the recommended approach. QEMU takes
> this
> > approach elsewhere (for example, please see x86_cpu_realizefn()), With
> > this change, pc_max_used_gpa() also uses the same method to detect 32-bit
> > processors.
> >
> > Unit tests are modified to not run 32-bit x86 tests that use memory
> hotplug.
> >
> > Suggested-by: David Hildenbrand <da...@redhat.com>
> > Signed-off-by: Ani Sinha <anisi...@redhat.com>
> > Reviewed-by: David Hildenbrand <da...@redhat.com>
> > ---
> > hw/i386/pc.c | 32 +++++++++++++++++++++++++++++---
> > hw/i386/pc_piix.c | 4 ++++
> > hw/i386/pc_q35.c | 2 ++
> > include/hw/i386/pc.h | 6 ++++++
> > tests/qtest/bios-tables-test.c | 26 ++++++++++++++++++--------
> > tests/qtest/numa-test.c | 7 ++++++-
> > 6 files changed, 65 insertions(+), 12 deletions(-)
> >
> > changelog:
> > v6: more code messaging. incorporated another of phil's suggestions.
>
> Thank you Ani, appreciated!
>
The code looks lot cleaner now. Thanks for the suggestions.
> v5: addressed phil's suggestions in code reorg to make it cleaner.
> > v4: address comments from v3. Fix a bug where compat knob was absent
> > from q35 machines. Commit message adjustment.
> > v3: still accounting for additional memory device region above 4G.
> > unit tests fixed (not running for 32-bit where mem hotplug is used).
> > v2: removed memory hotplug region from max_gpa. added compat knobs.
>
>
