Subject: Re: [PATCH v1] virt: arm-cca-guest: use raw variant of smp_processor_id() in arm_cca_report_new()

From: Catalin Marinas <[email-masked]>
Date: Mon, 18 May 2026 13:33:09 +0100

On Mon, May 18, 2026 at 12:31:31PM +0900, Kohei Enju wrote:
> With CONFIG_DEBUG_PREEMPT=y, smp_processor_id() becomes an alias of
> debug_smp_processor_id(). This debug function complains when certain
> conditions that ensure CPU ID stability are not met, specifically when
> it's called from a preemptible context.
> 
> In arm_cca_report_new(), which runs in a preemptible context,
> smp_processor_id() triggers a splat [0] due to this.
> 
> However, the CPU ID obtained here is used as the target CPU for
> smp_call_function_single() to designate a specific CPU for subsequent
> operations, not to assert that the current thread will continue to
> execute on the same CPU. Therefore, snapshotting the CPU ID itself is
> correct, and thus there's no actual harm except for the splat.
> 
> Use raw_smp_processor_id() instead, to directly retrieve the current CPU
> ID without the debug checks, avoiding the unnecessary warning message
> while preserving the correct functional behavior.
> 
> [0]
>  BUG: using smp_processor_id() in preemptible [00000000] code: cca-workload-at/134
>  caller is debug_smp_processor_id+0x20/0x2c
>  CPU: 0 UID: 0 PID: 134 Comm: cca-workload-at Not tainted 7.0.0-rc1-gc74a64d12073 #1 PREEMPT
>  Hardware name: linux,dummy-virt (DT)
>  Call trace:
>   [...]
>   check_preemption_disabled+0xf8/0x100
>   debug_smp_processor_id+0x20/0x2c
>   arm_cca_report_new+0x54/0x230
>   tsm_report_read+0x184/0x260
>   tsm_report_outblob_read+0x18/0x38
>   configfs_bin_read_iter+0xf4/0x1dc
>   vfs_read+0x230/0x31c
>   [...]
> 
> Fixes: 7999edc484ca ("virt: arm-cca-guest: TSM_REPORT support for realms")
> Signed-off-by: Kohei Enju <[email-masked]>
> ---
>  drivers/virt/coco/arm-cca-guest/arm-cca-guest.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c b/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c
> index 0c9ea24a200c..2d450caee3e4 100644
> --- a/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c
> +++ b/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c
> @@ -108,7 +108,7 @@ static int arm_cca_report_new(struct tsm_report *report, void *data)
>  	 * allocate outblob based on the returned value from the 'init'
>  	 * call and that cannot be done in an atomic context.
>  	 */
> -	cpu = smp_processor_id();
> +	cpu = raw_smp_processor_id();

That's just hiding the warning which might be genuine, irrespective of
what the comment says. Sashiko has some good points:

https://sashiko.dev/#/patchset/[email-masked]

Basically what guarantees that the cpu won't go offline? Can we use
migrate_disable() and ignore the smp_call_function_single() altogether?
It looks like a hack anyway.

We should also look at the other unrelated findings in this function
from Sashiko.

-- 
Catalin