Re: [Xen-devel] SMT/Hyperthreading detection not always correct

[Hans van Kranenburg <hans@xxxxxxxxxxx>]

On 09/03/2018 03:35 PM, Jan Beulich wrote:
>>>> On 03.09.18 at 15:24, <hans@xxxxxxxxxxx> wrote:
>> (XEN) ACPI: LAPIC (acpi_id[0x00] lapic_id[0x00] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x10] lapic_id[0x20] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x08] lapic_id[0x10] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x18] lapic_id[0x30] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x04] lapic_id[0x04] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x14] lapic_id[0x24] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x0c] lapic_id[0x14] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x1c] lapic_id[0x34] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x02] lapic_id[0x02] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x12] lapic_id[0x22] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x0a] lapic_id[0x12] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x1a] lapic_id[0x32] enabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x06] lapic_id[0x06] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x16] lapic_id[0x26] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x0e] lapic_id[0x16] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x1e] lapic_id[0x36] disabled)
> 
> These look to be all the primary threads, the last few disabled due
> to being absent.
> 
>> (XEN) ACPI: LAPIC (acpi_id[0x01] lapic_id[0x01] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x11] lapic_id[0x21] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x09] lapic_id[0x11] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x19] lapic_id[0x31] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x05] lapic_id[0x05] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x15] lapic_id[0x25] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x0d] lapic_id[0x15] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x1d] lapic_id[0x35] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x03] lapic_id[0x03] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x13] lapic_id[0x23] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x0b] lapic_id[0x13] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x1b] lapic_id[0x33] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x07] lapic_id[0x07] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x17] lapic_id[0x27] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x0f] lapic_id[0x17] disabled)
>> (XEN) ACPI: LAPIC (acpi_id[0x1f] lapic_id[0x37] disabled)
> 
> And these are all the secondary threads afaict, which are all
> disabled. I'm unconvinced we can assume a system is not using
> hyperthreading simply based upon their MADT entries showing all
> of them as disabled - what if a HT-enabled CPU was later
> hotplugged?
> 
> I would have hoped that the topology related CPUID output
> would change when HT is (properly) disabled by the BIOS.
> 
> For the moment I can't see a way around you specifying "smt=0"
> on systems like this one.

The actual reason why I started the conversation about it was that for a
regular end user, the message telling the user that he has "hardware
with SMT/Hyperthreading enabled" is a confusing thing if you just made
sure that you have it disabled in the settings.

So, alternatively, instead of a technical solution, the warning could be
changed to something like "Booted on L1TF-vulnerable hardware and unable
to properly detect if SMT/Hyperthreading is enabled or disabled....
Please choose explicit smt setting blah to make sure that it behaves how
you want."

As a user, that's less confusing, it just tells me that Xen likes me to
make explicit what I want.

Hans

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