Re: [PATCH 1/3] ACPI: APEI: GHES: Improve ghes_notify_nmi() status check

[Hanjun Guo]

Hi Shuai,

Some minor comments inline.

On 2025/12/3 21:02, Shuai Xue wrote:
> From: Tony Luck <tony.luck@xxxxxxxxx>
>
> 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.
>
> 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.
>
> 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>
> ---
>   drivers/acpi/apei/ghes.c | 39 ++++++++++++++++++++++++++++++++++++---
>   include/acpi/ghes.h      |  1 +
>   2 files changed, 37 insertions(+), 3 deletions(-)
>
> 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;
>   
>   	if (!atomic_add_unless(&ghes_in_nmi, 1, 1))
>   		return ret;
> @@ -1439,13 +1453,26 @@ static int ghes_notify_nmi(unsigned int cmd, struct pt_regs *regs)
>   	return ret;
>   }
>   
> -static void ghes_nmi_add(struct ghes *ghes)
> +static int ghes_nmi_add(struct ghes *ghes)
>   {
> +	struct acpi_hest_generic *g = ghes->generic;
> +	u64 paddr;
> +	int rc;
> +
> +	rc = apei_read(&paddr, &g->error_status_address);
> +	if (rc)
> +		return rc;

It will be good to add a empty line here.

> +	ghes->error_status_vaddr = acpi_os_ioremap(paddr, sizeof(ghes->estatus->block_status));
> +	if (!ghes->error_status_vaddr)
> +		return AE_BAD_ADDRESS;

It's static int for ghes_nmi_add(), and AE_BAD_ADDRESS is the type of
acpi_status, it's better to return -EINVAL here.

Thanks
Hanjun