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

[Jan Beulich <jbeulich@suse.com>]

On 23.02.2026 20:56, David Thomson wrote:
> 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>
> ---
>  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 2967039..67a4afc 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 (!acpi_has_method(handle, "_CST") && !pblk)
> +		return AE_OK;

I understand you reflect original behavior in this regard, but why involve any
ACPI function here at all when pblk is non-zero? I.e. why not swap the operands
of && ? Object evaluation could have wanted side effects (in which case,
however, some different change would be needed here), but checking for method
presence surely hasn't.

Jan