xen/acpi-processor: fix _CST detection using undersized evaluation buffer

read_acpi_id() attempts to evaluate _CST using a stack buffer of
sizeof(union acpi_object) (48 bytes), but _CST returns a nested Package
of sub-Packages (one per C-state, each containing a register descriptor,
type, latency, and power) requiring hundreds of bytes. The evaluation
always fails with AE_BUFFER_OVERFLOW.

On modern systems using FFH/MWAIT entry (where pblk is zero), this
causes the function to return before setting the acpi_id_cst_present
bit. In check_acpi_ids(), flags.power is then zero for all Phase 2 CPUs
(physical CPUs beyond dom0's vCPU count), so push_cxx_to_hypervisor() is
never called for them.

On a system with dom0_max_vcpus=2 and 8 physical CPUs, only PCPUs 0-1
receive C-state data. PCPUs 2-7 are stuck in C0/C1 idle, unable to
enter C2/C3. This costs measurable wall power (4W observed on an Intel
Core Ultra 7 265K with Xen 4.20).

The function never uses the _CST return value -- it only needs to know
whether _CST exists. Replace the broken acpi_evaluate_object() call with
acpi_has_method(), which correctly detects _CST presence using
acpi_get_handle() without any buffer allocation. This brings C-state
detection to parity with the P-state path, which already works correctly
for Phase 2 CPUs.

Fixes: 59a56802 ("xen/acpi-processor: C and P-state driver that uploads said data to hypervisor.")
Signed-off-by: David Thomson <dt@linux-mail.net>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20260224093707.19679-1-dt@linux-mail.net>