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Xen C-state Issues

ACPI C-state support might not see too much use, but it does see some.

With Xen 4.11 and Linux kernel 4.19, I found higher C-states only got
enabled for physical cores for which Domain 0 had a corresponding vCPU.
On a machine where Domain 0 has 5 vCPUs, but 8 reported cores, the
additional C-states would only be enabled on cores 0-4.

This can be worked around by giving Domain 0 vCPUs equal to cores, but
then offlining the extra vCPUs.  I'm guessing this is a bug with the
Linux 4.19 xen_acpi_processor module.

Appears Xen 4.14 doesn't work at all with Linux kernel 4.19's ACPI
C-state support.  This combination is unable to enable higher C-states
on any core.  Since Xen 4.14 and Linux 4.19 are *both* *presently*
supported it seems patch(es) are needed somewhere for this combination.

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