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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel