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Date:   Mon, 1 Aug 2022 14:20:39 -0700
From:   Dan Williams <dan.j.williams@intel.com>
To:     Jane Chu <jane.chu@oracle.com>,
        Dan Williams <dan.j.williams@intel.com>,
        "tony.luck@intel.com" <tony.luck@intel.com>,
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        "hch@lst.de" <hch@lst.de>,
        "nvdimm@lists.linux.dev" <nvdimm@lists.linux.dev>
Subject: Re: [PATCH v5] x86/mce: retrieve poison range from hardware
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Jane Chu wrote:
> On 8/1/2022 9:44 AM, Dan Williams wrote:
> > 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.
> 
> Thanks Dan!  However it looks like find_first_bit() could call into 
> __ffs(x) which has the same limitation as __ffs64(x), as Tony pointed out.

Not quite, no. __ffs() behavior is *undefined* if the input is zero.
find_first_bit() is *defined* and returns @size is the input is zero.
Which is the behavior this wants to default to PAGE_SHIFT in the absence
of any smaller granularity information.