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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 11, 2024 at 09:48:22AM +0200, 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)"

Maybe it would be better to just make this a boolean,
and put the spec related logic in smbios.c ?
WDYT?

>   */
>  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;
>  };
>  
>  /**
> 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)
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index bc38cad7f2..3cfdaec65d 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -1004,6 +1004,7 @@ static void machine_class_init(ObjectClass *oc, void *data)
>      /* Default 128 MB as guest ram size */
>      mc->default_ram_size = 128 * MiB;
>      mc->rom_file_has_mr = true;
> +    mc->smbios_memory_device_size = 2047 * TiB;
>  
>      /* numa node memory size aligned on 8MB by default.
>       * On Linux, each node's border has to be 8MB aligned



All these values really should be documented.
And I feel 



> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> index 9445b07b4f..d9e69243b4 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -495,6 +495,7 @@ static void pc_i440fx_machine_9_0_options(MachineClass *m)
>      pc_i440fx_machine_9_1_options(m);
>      m->alias = NULL;
>      m->is_default = false;
> +    m->smbios_memory_device_size = 16 * GiB;
>  
>      compat_props_add(m->compat_props, hw_compat_9_0, hw_compat_9_0_len);
>      compat_props_add(m->compat_props, pc_compat_9_0, pc_compat_9_0_len);
> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> index 71d3c6d122..9d108b194e 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -374,6 +374,7 @@ static void pc_q35_machine_9_0_options(MachineClass *m)
>      PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
>      pc_q35_machine_9_1_options(m);
>      m->alias = NULL;
> +    m->smbios_memory_device_size = 16 * GiB;
>      compat_props_add(m->compat_props, hw_compat_9_0, hw_compat_9_0_len);
>      compat_props_add(m->compat_props, pc_compat_9_0, pc_compat_9_0_len);
>      pcmc->isa_bios_alias = false;
> diff --git a/hw/smbios/smbios.c b/hw/smbios/smbios.c
> index 3b7703489d..a394514264 100644
> --- a/hw/smbios/smbios.c
> +++ b/hw/smbios/smbios.c
> @@ -1093,6 +1093,7 @@ static bool smbios_get_tables_ep(MachineState *ms,
>                         Error **errp)
>  {
>      unsigned i, dimm_cnt, offset;
> +    MachineClass *mc = MACHINE_GET_CLASS(ms);
>      ERRP_GUARD();
>  
>      assert(ep_type == SMBIOS_ENTRY_POINT_TYPE_32 ||
> @@ -1123,12 +1124,12 @@ static bool smbios_get_tables_ep(MachineState *ms,
>      smbios_build_type_9_table(errp);
>      smbios_build_type_11_table();
>  
> -#define MAX_DIMM_SZ (16 * GiB)
> -#define GET_DIMM_SZ ((i < dimm_cnt - 1) ? MAX_DIMM_SZ \
> -                                        : ((current_machine->ram_size - 1) % MAX_DIMM_SZ) + 1)
> +#define GET_DIMM_SZ ((i < dimm_cnt - 1) ? mc->smbios_memory_device_size \
> +    : ((current_machine->ram_size - 1) % mc->smbios_memory_device_size) + 1)
>  
> -    dimm_cnt = QEMU_ALIGN_UP(current_machine->ram_size, MAX_DIMM_SZ) /
> -               MAX_DIMM_SZ;
> +    dimm_cnt = QEMU_ALIGN_UP(current_machine->ram_size,
> +                             mc->smbios_memory_device_size) /
> +               mc->smbios_memory_device_size;
>  
>      /*
>       * The offset determines if we need to keep additional space between
> -- 
> 2.43.0