Re: [PATCH] x86/ACPI/boot: Use FADT version to check support for online capable

["Zhang, Rui" <rui.zhang@intel.com>]

On Wed, 2023-03-29 at 12:45 -0500, Mario Limonciello wrote:
> ACPI 6.3 introduced the online capable bit, and also introduced MADT
> version 5.
> 
> This was used to distinguish whether the offset storing online
> capable
> could be used. However ACPI 6.2b has MADT version "45" which is for
> an errata version of the ACPI 6.2 spec.

I made a double check.

In https://uefi.org/sites/default/files/resources/ACPI_6_2.pdf,
MADT revision is 4.

In 
https://uefi.org/sites/default/files/resources/ACPI%206_2_A_Sept29.pdf,
MADT revision is 45.

In 
https://uefi.org/sites/default/files/resources/ACPI_6_2_B_final_Jan30.pdf
MADT revision is 5.

So you probably mean 6.2a has MADT revision "45" here?

>   This means that the Linux code
> for detecting availability of MADT will mistakingly flag ACPI 6.2b as
> supporting online capable which is inaccurate as it's an ACPI 6.3
> feature.
> 
> Instead use the FADT major and minor revision fields to distingush
> this.
> 
> Reported-by: Eric DeVolder <eric.devolder@oracle.com>
> Reported-by: Borislav Petkob <bp@alien8.de>
> Fixes: aa06e20f1be6 ("x86/ACPI: Don't add CPUs that are not online
> capable")
> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> ---
>  arch/x86/kernel/acpi/boot.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/acpi/boot.c
> b/arch/x86/kernel/acpi/boot.c
> index 1c38174b5f01..e92e3292fef7 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -146,7 +146,10 @@ static int __init acpi_parse_madt(struct
> acpi_table_header *table)
>  
>  		pr_debug("Local APIC address 0x%08x\n", madt->address);
>  	}
> -	if (madt->header.revision >= 5)
> +
> +	if (acpi_gbl_FADT.header.revision > 6 ||
> +	    (acpi_gbl_FADT.header.revision == 6 &&
> +	     acpi_gbl_FADT.minor_revision >= 3))
>  		acpi_support_online_capable = true;

Better to have a comment here?
For me, it is hard to understand this by reading the code directly.

thanks,
rui