Re: [PATCH] ACPI: APEI: GHES: Improve ghes_notify_nmi() status check

[Shuai Xue]

在 2025/11/4 07:05, Tony Luck 写道:
> ghes_notify_nmi() is called for every NMI and must check whether the NMI was
> generated because an error was signalled by platform firmware.
>
> This check is very expensive as for each registered GHES NMI source it reads
> from the acpi generic address attached to this error source to get the physical
> address of the acpi_hest_generic_status block.  It then checks the "block_status"
> to see if an error was logged.
>
> The ACPI/APEI code must create virtual mappings for each of those physical
> addresses, and tear them down afterwards. On an Icelake system this takes around
> 15,000 TSC cycles. Enough to disturb efforts to profile system performance.

Hi, Tony

Interesting.

If I understand correctly, you mean ghes_peek_estatus() and
ghes_clear_estatus().

I conducted performance testing on our system (ARM v8) and found the
following average costs:

- ghes_peek_estatus(): 8,138.3 ns (21,160 cycles)
- ghes_clear_estatus(): 2,038.3 ns (5,300 cycles)

> If that were not bad enough, there are some atomic accesses in the code path
> that will cause cache line bounces between CPUs. A problem that gets worse as
> the core count increases.

Could you elaborate on which specific atomic accesses you're referring to?

> But BIOS changes neither the acpi generic address nor the physical address of
> the acpi_hest_generic_status block. So this walk can be done once when the NMI is
> registered to save the virtual address (unmapping if the NMI is ever unregistered).
> The "block_status" can be checked directly in the NMI handler. This can be done
> without any atomic accesses.
>
> Resulting time to check that there is not an error record is around 900 cycles.
>
> Reported-by: Andi Kleen <andi.kleen@xxxxxxxxx>
> Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
>
> ---
> N.B. I only talked to an Intel BIOS expert about this. GHES code is shared by
> other architectures, so it would be wise to get confirmation on whether this
> assumption applies to all, or is Intel (or X86) specific.

The assumption is "BIOS changes neither the acpi generic address nor the
physical address of the acpi_hest_generic_status block."?

I've consulted with our BIOS experts from both ARM and RISC-V platform
teams, and they confirmed that error status blocks are reserved at boot
time and remain unchanged during runtime.

> ---
>   include/acpi/ghes.h      |  1 +
>   drivers/acpi/apei/ghes.c | 39 ++++++++++++++++++++++++++++++++++++---
>   2 files changed, 37 insertions(+), 3 deletions(-)
>
> diff --git a/include/acpi/ghes.h b/include/acpi/ghes.h
> index ebd21b05fe6e..58655d313a1f 100644
> --- a/include/acpi/ghes.h
> +++ b/include/acpi/ghes.h
> @@ -29,6 +29,7 @@ struct ghes {
>   	};
>   	struct device *dev;
>   	struct list_head elist;
> +	void __iomem *error_status_vaddr;
>   };
>   
>   struct ghes_estatus_node {
> diff --git a/drivers/acpi/apei/ghes.c b/drivers/acpi/apei/ghes.c
> index 97ee19f2cae0..62713b612865 100644
> --- a/drivers/acpi/apei/ghes.c
> +++ b/drivers/acpi/apei/ghes.c
> @@ -1425,7 +1425,21 @@ static LIST_HEAD(ghes_nmi);
>   static int ghes_notify_nmi(unsigned int cmd, struct pt_regs *regs)
>   {
>   	static DEFINE_RAW_SPINLOCK(ghes_notify_lock_nmi);
> +	bool active_error = false;
>   	int ret = NMI_DONE;
> +	struct ghes *ghes;
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(ghes, &ghes_nmi, list) {
> +		if (ghes->error_status_vaddr && readl(ghes->error_status_vaddr)) {
> +			active_error = true;
> +			break;
> +		}
> +	}
> +	rcu_read_unlock();
> +
> +	if (!active_error)
> +		return ret;

Shoud we put active_error into struct ghes? If we know it is active, we
do not need to call __ghes_peek_estatus() to estatus->block_status().

Thanks.
Shuai