Re: [PATCH v3] x86/mce: Do not treat cache hierarchy errors as memory errors

[Yazen Ghannam]

On Mon, Oct 12, 2026 at 09:33:05PM +0200, Adrian Schlegel wrote:
> On Intel and Zhaoxin, mce_is_memory_error() also returns true for cache
> hierarchy errors. All callers expect it to report memory errors only.
> 
> The CEC uses it to count corrected DRAM errors. A corrected cache error
> only names the cache line that was affected, not a faulty DRAM page.
> With a faulty CPU core, the CEC therefore soft-offlines healthy pages.
> On an i9-9900K this caused about 500 soft-offline attempts per hour.
> 
> Report only memory controller errors on Intel and Zhaoxin, as is
> already done for AMD.
> 
> Fixes: fa92c5869426 ("x86, mce: Support memory error recovery for both UCNA and Deferred error in machine_check_poll")
> Signed-off-by: Adrian Schlegel <me@xxxxxxxxxxxxxxxxxx>
> ---
> v3:
> - Shorten and wrap commit message (Boris, Tony)
> - Refer to the SDM section by name instead of number (Tony)
> 
> v2:
> - Fix mce_is_memory_error() itself instead of adding a CEC-local helper (Tony)
> - v1: https://lore.kernel.org/r/20260927151904.30524-1-me@xxxxxxxxxxxxxxxxxx
> 
> Tested with mce-inject (sw) in a VM with an Intel CPU model. A corrected
> cache error (status 0xcc5ffc0000100179) is no longer counted and its
> page stays online. A corrected memory controller error (status
> 0x8c0000000000009f) is still counted and its page soft-offlined.
> 
>  arch/x86/kernel/cpu/mce/core.c | 18 +++++-------------
>  1 file changed, 5 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
> index ab469605f..54ed053fa 100644
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -540,21 +540,13 @@ bool mce_is_memory_error(struct mce *m)
>  	case X86_VENDOR_INTEL:
>  	case X86_VENDOR_ZHAOXIN:
>  		/*
> -		 * Intel SDM Volume 3B - 15.9.2 Compound Error Codes
> +		 * Intel SDM Volume 3B "Compound Error Codes".
>  		 *
> -		 * Bit 7 of the MCACOD field of IA32_MCi_STATUS is used for
> -		 * indicating a memory error. Bit 8 is used for indicating a
> -		 * cache hierarchy error. The combination of bit 2 and bit 3
> -		 * is used for indicating a `generic' cache hierarchy error
> -		 * But we can't just blindly check the above bits, because if
> -		 * bit 11 is set, then it is a bus/interconnect error - and
> -		 * either way the above bits just gives more detail on what
> -		 * bus/interconnect error happened. Note that bit 12 can be
> -		 * ignored, as it's the "filter" bit.
> +		 * Memory controller errors have an MCACOD of 000F 0000 1MMM CCCC:
> +		 * bit 7 set, bits 8-11 and 13-15 clear. Bit 12 is the "filter"
> +		 * bit and is ignored.
>  		 */
> -		return (m->status & 0xef80) == BIT(7) ||
> -		       (m->status & 0xef00) == BIT(8) ||
> -		       (m->status & 0xeffc) == 0xc;
> +		return (m->status & 0xef80) == BIT(7);
>  

The SDM also mentions "Extended Memory Errors". I guess these were left
out before, but should they be included?

Thanks,
Yazen