On Tue, Jan 16, 2024 at 03:25:47PM +0000, Michael Kelley wrote:

From: Saurabh Sengar <ssengar@linux.microsoft.com> Sent: Monday, January 15, 2024 9:58 AM

quoted

The current method for signaling the compatibility of a Hyper-V host
with MSIs featuring 15-bit APIC IDs relies on a synthetic cpuid leaf.
However, for higher VTLs, this leaf is not reported, due to the absence
of an IO-APIC.

As an alternative, assume that when running at a high VTL, the host
supports 15-bit APIC IDs. This assumption is safe, as Hyper-V does not
employ any architectural MSIs at higher VTLs

This unblocks startup of VTL2 environments with more than 256 CPUs.

Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>

[...]

Reviewed-by: Michael Kelley <redacted>

Applied. Thanks.