RE: [PATCH v5] x86/mce: retrieve poison range from hardware

[Dan Williams <dan.j.williams@intel.com>]

Jane Chu wrote:
> With Commit 7917f9cdb503 ("acpi/nfit: rely on mce->misc to determine
> poison granularity") that changed nfit_handle_mce() callback to report
> badrange according to 1ULL << MCI_MISC_ADDR_LSB(mce->misc), it's been
> discovered that the mce->misc LSB field is 0x1000 bytes, hence injecting
> 2 back-to-back poisons and the driver ends up logging 8 badblocks,
> because 0x1000 bytes is 8 512-byte.
> 
> Dan Williams noticed that apei_mce_report_mem_error() hardcode
> the LSB field to PAGE_SHIFT instead of consulting the input
> struct cper_sec_mem_err record.  So change to rely on hardware whenever
> support is available.
> 
> Link: https://lore.kernel.org/r/7ed50fd8-521e-cade-77b1-738b8bfb8502@oracle.com
> 
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Jane Chu <jane.chu@oracle.com>
> ---
>  arch/x86/kernel/cpu/mce/apei.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/mce/apei.c b/arch/x86/kernel/cpu/mce/apei.c
> index 717192915f28..2c7ea0ba9dd7 100644
> --- a/arch/x86/kernel/cpu/mce/apei.c
> +++ b/arch/x86/kernel/cpu/mce/apei.c
> @@ -29,15 +29,27 @@
>  void apei_mce_report_mem_error(int severity, struct cper_sec_mem_err *mem_err)
>  {
>  	struct mce m;
> +	int lsb = PAGE_SHIFT;
>  
>  	if (!(mem_err->validation_bits & CPER_MEM_VALID_PA))
>  		return;
>  
> +	/*
> +	 * Even if the ->validation_bits are set for address mask,
> +	 * to be extra safe, check and reject an error radius '0',
> +	 * and fallback to the default page size.
> +	 */
> +	if (mem_err->validation_bits & CPER_MEM_VALID_PA_MASK) {
> +		lsb = __ffs64(mem_err->physical_addr_mask);
> +		if (lsb == 1)

This was the reason I recommended hweight64 and min_not_zero() as
hweight64 does not have the undefined behavior. However, an even better
option is to just do:

    find_first_bit(&mem_err->physical_addr_mask, PAGE_SHIFT)

...as that trims the result to the PAGE_SHIFT max and handles the zero
case.