Re: [PATCH v2] xen/acpi-processor: fix _CST detection using undersized evaluation buffer

[Elliott Mitchell <ehem+xen@m5p.com>]

On Tue, Feb 24, 2026 at 09:37:11AM +0000, David Thomson wrote:
> 
> 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>

Tested-by: Elliott Mitchell <ehem+xen@m5p.com>

The window of opportunity happened and I confirmed it fixed the issue for
me.  I don't know whether this already got in though.

> -	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);

I had traced the problem to this spot a while back, but I didn't know
what an appropriate change was.  Since I had a workaround this wasn't
urgent.  I do know this effects many others, I don't know how urgent this
bug is for everyone else.


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