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Message-ID: <314eedc5-c27e-4e63-b74a-7b06f64fdd86@intel.com>
Date: Sat, 16 Dec 2023 04:41:36 +0530
User-Agent: Mozilla Thunderbird
Subject: Re: x86/mce: Is mce_is_memory_error() incorrect for Intel?
To: "Luck, Tony" <tony.luck@intel.com>, "x86@kernel.org" <x86@kernel.org>,
	Borislav Petkov <bp@alien8.de>
CC: Thomas Gleixner <tglx@linutronix.de>, Peter Zijlstra
	<peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>, Dave Hansen
	<dave.hansen@linux.intel.com>, "H . Peter Anvin" <hpa@zytor.com>, "Yazen
 Ghannam" <yazen.ghannam@amd.com>, Arnd Bergmann <arnd@arndb.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>
References: <20231206013846.1859347-1-sohil.mehta@intel.com>
 <4ce89a5e-96f8-4939-b86e-f65c16f4bd4e@intel.com>
 <SJ1PR11MB60831E048B544C83E845D907FC8DA@SJ1PR11MB6083.namprd11.prod.outlook.com>
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From: Sohil Mehta <sohil.mehta@intel.com>
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Thanks Tony for the explanation. It is very helpful.

>> Type                          Form
>> ----                          ----
>> Generic Cache Hierarchy       000F 0000 0000 11LL
>> TLB Errors                    000F 0000 0001 TTLL
>> Memory Controller Errors      000F 0000 1MMM CCCC
>> Cache Hierarchy Errors                000F 0001 RRRR TTLL
>> Extended Memory Errors                000F 0010 1MMM CCCC
>> Bus and Interconnect Errors   000F 1PPT RRRR IILL
>>
>> I am not sure what are the practical implications of getting
>> mce_is_memory_error() wrong. (This issue is completely theoretical right
>> now.) Any insights?
> 
> This function is used to check whether an address is OS addressable memory
> (i.e. for a page that could be taken offline). That doesn't apply to the caching
> use case (the only way to "offline" such a page would be to offline each of the
> slow memory pages that it might be used for).
> 

Makes sense. I am assuming these Extended Memory Errors will not be used
anymore (even for CXL.mem type configs) and we don't need to include
them in the mce_is_memory_error() check? I'll update the comment
accordingly.

> I'm not quite sure why bit 8 (cache hierarchy error) was added into this check,
> It would seem to have the same issues as extended memory.
> 

From a little bit of digging it seems the check for "cache hierarchy
errors" was always there. Commit fa92c5869426 ("x86, mce: Support memory
error recovery for both UCNA and Deferred error in machine_check_poll")
introduced the original checks but maybe the intention at that time was
different? I see that the CEC stuff was added later so maybe the
original memory related failures were handled differently?

Now, should we remove the cache error related check from
mce_is_memory_error()?