[PATCH 5.15 077/570] xen/acpi-processor: fix _CST detection using undersized evaluation buffer

Greg Kroah-Hartman
2026-04-13T16:21:25Z

From: David Thomson <dt@linux-mail.net>

[ Upstream commit 8b57227d59a86fc06d4f09de08f98133680f2cae ]

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: 59a568029181 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/xen/xen-acpi-processor.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/xen/xen-acpi-processor.c b/drivers/xen/xen-acpi-processor.c
index df7cab870be5a..aa22c2f52bae7 100644
--- a/drivers/xen/xen-acpi-processor.c
+++ b/drivers/xen/xen-acpi-processor.c
@@ -379,11 +379,8 @@ read_acpi_id(acpi_handle handle, u32 lvl, void *context, void **rv)
acpi_psd[acpi_id].domain);
}

- status = acpi_evaluate_object(handle, "_CST", NULL, &buffer);
- if (ACPI_FAILURE(status)) {
- if (!pblk)
- return AE_OK;
- }
+ if (!pblk && !acpi_has_method(handle, "_CST"))
+ return AE_OK;
/* .. and it has a C-state */
__set_bit(acpi_id, acpi_id_cst_present);

--
2.51.0