Re: [PATCH] smbios: make memory device size configurable per Machine

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 11/7/24 10:42, Igor Mammedov wrote:
> On Thu, 11 Jul 2024 10:19:27 +0200
> Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
> 
>> Hi Igor,
>>
>> On 11/7/24 09:48, Igor Mammedov wrote:
>>> Currently SMBIOS maximum memory device chunk is capped at 16Gb,
>>> which is fine for the most cases (QEMU uses it to describe initial
>>> RAM (type 17 SMBIOS table entries)).
>>> However when starting guest with terabytes of RAM this leads to
>>> too many memory device structures, which eventually upsets linux
>>> kernel as it reserves only 64K for these entries and when that
>>> border is crossed out it runs out of reserved memory.
>>>
>>> Instead of partitioning initial RAM on 16Gb chunks, use maximum
>>> possible chunk size that SMBIOS spec allows[1]. Which lets
>>> encode RAM in Mb units in uint32_t-1 field (upto 2047Tb).
>>> As result initial RAM will generate only one type 17 structure
>>> until host/guest reach ability to use more RAM in the future.
>>>
>>> Compat changes:
>>> We can't unconditionally change chunk size as it will break
>>> QEMU<->guest ABI (and migration). Thus introduce a new machine class
>>> field that would let older versioned machines to use 16Gb chunks
>>> while new machine type could use maximum possible chunk size.
>>>
>>> While it might seem to be risky to rise max entry size this much
>>> (much beyond of what current physical RAM modules support),
>>> I'd not expect it causing much issues, modulo uncovering bugs
>>> in software running within guest. And those should be fixed
>>> on guest side to handle SMBIOS spec properly, especially if
>>> guest is expected to support so huge RAM configs.
>>> In worst case, QEMU can reduce chunk size later if we would
>>> care enough about introducing a workaround for some 'unfixable'
>>> guest OS, either by fixing up the next machine type or
>>> giving users a CLI option to customize it.
>>>
>>> 1) SMBIOS 3.1.0 7.18.5 Memory Device — Extended Size
>>>
>>> PS:
>>> * tested on 8Tb host with RHEL6 guest, which seems to parse
>>>     type 17 SMBIOS table entries correctly (according to 'dmidecode').
>>>
>>> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
>>> ---
>>>    include/hw/boards.h |  4 ++++
>>>    hw/arm/virt.c       |  1 +
>>>    hw/core/machine.c   |  1 +
>>>    hw/i386/pc_piix.c   |  1 +
>>>    hw/i386/pc_q35.c    |  1 +
>>>    hw/smbios/smbios.c  | 11 ++++++-----
>>>    6 files changed, 14 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/include/hw/boards.h b/include/hw/boards.h
>>> index ef6f18f2c1..48ff6d8b93 100644
>>> --- a/include/hw/boards.h
>>> +++ b/include/hw/boards.h
>>> @@ -237,6 +237,9 @@ typedef struct {
>>>     *    purposes only.
>>>     *    Applies only to default memory backend, i.e., explicit memory backend
>>>     *    wasn't used.
>>> + * @smbios_memory_device_size:
>>> + *    Default size of memory device,
>>> + *    SMBIOS 3.1.0 "7.18 Memory Device (Type 17)"
>>>     */
>>>    struct MachineClass {
>>>        /*< private >*/
>>> @@ -304,6 +307,7 @@ struct MachineClass {
>>>        const CPUArchIdList *(*possible_cpu_arch_ids)(MachineState *machine);
>>>        int64_t (*get_default_cpu_node_id)(const MachineState *ms, int idx);
>>>        ram_addr_t (*fixup_ram_size)(ram_addr_t size);
>>> +    uint64_t smbios_memory_device_size;
>>
>> Quick notes since I'm on holidays (not meant to block this patch):
>>
>> - How will evolve this machine class property in the context of
>>     a heterogeneous machine (i.e. x86_64 cores and 1 riscv32 one)?
> 
> I'm not aware of a SMBIOS spec (3.x) that cares about that heterogeneous
> setup yet. Are there anything in that area exists yet?

Not yet.

> 
>> - Should this become a SmbiosProviderInterface later?
> if/when SMBIOS does get there (heterogeneous machines), introducing
> an interface might make a sense.

OK.