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

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

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)?

- Should this become a SmbiosProviderInterface later?

>   };
>   
>   /**
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index b0c68d66a3..719e83e6a1 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -3308,6 +3308,7 @@ DEFINE_VIRT_MACHINE_AS_LATEST(9, 1)
>   static void virt_machine_9_0_options(MachineClass *mc)
>   {
>       virt_machine_9_1_options(mc);
> +    mc->smbios_memory_device_size = 16 * GiB;
>       compat_props_add(mc->compat_props, hw_compat_9_0, hw_compat_9_0_len);
>   }
>   DEFINE_VIRT_MACHINE(9, 0)

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