Re: [PATCH v2] pc: q35: Bump max_cpus to 1024

["Suthikulpanit, Suravee" <suravee.suthikulpanit@amd.com>]

On 6/1/2023 6:09 PM, Michael S. Tsirkin wrote:
> On Thu, Jun 01, 2023 at 11:17:30AM +0100, Daniel P. Berrangé wrote:
>> On Thu, Jun 01, 2023 at 11:09:45AM +0100, Joao Martins wrote:
>>>
>>> On 31/05/2023 23:51, Suravee Suthikulpanit wrote:
>>>> Since KVM_MAX_VCPUS is currently defined to 1024 for x86 as shown in
>>>> arch/x86/include/asm/kvm_host.h, update QEMU limits to the same number.
>>>>
>>>> In case KVM could not support the specified number of vcpus, QEMU would
>>>> return the following error message:
>>>>
>>>>    qemu-system-x86_64: kvm_init_vcpu: kvm_get_vcpu failed (xxx): Invalid argument
>>>>
>>>> Signed-off-by: Suravee Suthikulpanit<suravee.suthikulpanit@amd.com>
>>>> ---
>>>>
>>>> Changes from V1:
>>>> (https://lore.kernel.org/all/YnkDGsIi1vFvXmiP@redhat.com/T/)
>>>>   * Bump from 512 to KVM_MAX_VCPUS (per Igor's suggestion)
>>>>
>>>> Note:
>>>>   From the last discussion, Daniel mentioned that SMBIO 2.1 tables might
>>>>   cause overflow at approx 720 CPUs, and it might require using the
>>>>   SMBIO 3.0 entry point. Also, we might need to change the default for
>>>>   the x86 machine type to SMBIO 3.0. However, I do not know the status
>>>>   of this.
>>>>
>>> I suspect smbios 3.0 (64-bit entry point) is already supported.
>>>
>>> With current qemu and all the smbios fixes in the last cycle, perhaps this is
>>> mainly just setting smbios_entry_point_type to SMBIOS_ENTRY_POINT_TYPE_64 if
>>> MachineState::smp::max_cpus is bigger than 720 (e.g. in pc_q35_init()?)
>> >> The need for the 64-bit entry point depends on a combination of RAM 
config
>> and CPU count. IMHO we need to unconditionally switch the latest machine
>> types to use the 64-bit entry point by default, rather than trying to infer
>> some special condition to dynamically change on the fly.
> Yes, makes sense.

Thanks all for the feedback. So, IIUC, here is how the SMBIOS entry 
point types would be affected by the QEMU options:

- pc-q35-8.1 and later, default to SMBIOS EP type 64.
- pc-q35-8.0 and older, default to SMBIOS EP type 32.
- User can override the type w/ QEMU option "-M .., 
smbios-entry-point-type=[32|64]"

Please let me know if I am missing anything. If this is accurate, I'll 
send out v3 with this change.

Thanks,
Suravee