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

[Igor Mammedov <imammedo@redhat.com>]

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?

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

> 
> >   };
> >   
> >   /**
> > 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)  
> 
> [...]
>