Re: [PATCH] mce: prevent concurrent polling of MCE events

[Yazen Ghannam <yazen.ghannam@amd.com>]

On 6/9/23 11:24 AM, Luck, Tony wrote:
>> So "UCNA" is like the AMD "Deferred" severity it seems. How is this
>> different from "Action Optional"? I've been equating DFR and AO.
>

Thanks Tony.

> Categories of uncorrected errors memory errors on Intel
> 
> 1) "UCNA" ... these are logged by memory controllers when ECC says that a memory read cannot
> supply correct data. If CMCI is enabled, signaled with CMCI. Note that these will occur on prefetch
> or speculative reads as well as "regular" reads. The data might never be consumed.
>

Yes, this is like AMD.

Key differences:
	* Logged using "Deferred" severity. However, deferred errors
	  aren't always from the memory controller. So there still needs
	  to be an error code check in addition to severity.
	* Signalled with a Deferred error APIC interrupt. This way UC
	  errors can be signalled independently of CEs.

> 2) "SRAO". This is now legacy. Pre-Icelake systems log these for uncorrected errors found by
> the patrol scrubber, and for evictions of poison from L3 cache (if that poison was due to an ECC
> failure in the cache itself, not for poison created elsewhere and currently resident in the cache).
> Signaled with a broadcast machine check. Icelake and newer systems use UCNA for these.
>

Yes, this mostly fails within the Deferred/UCNA case for AMD also.

> 3) "SRAR". Attempt to consume poison (either data read or instruction fetch). Signaled with
> machine check. Pre-Skylake this was broadcast. Skylake and newer have an opt-in mechanism
> to request #MC delivery to just the logical CPU trying to consume (Linux always opts-in).
> 
> 
> UCNA = Uncorrected No Action required. But Linux does take action to try to offline the page.
>

That's right. So we'll use this for the Deferred memory error case. But
there will need to be updates for these to be actionable on AMD systems.

> SRAO = Software Recoverable Action Optional. As with UCNA Linux tries to offline the page.
>

Okay, so ignore these going forward.

> SRAR = Software Recoverable Action Required. Linux will replace a clean page with a new copy
> if it can (think read-only text pages mapped from ELF executable). If not it sends SIGBUS to the
> application. Some SRAR in the kernel are recoverable ... see the copy_mc*() functions.
>

Yep, this mostly works. There's still an AMD IF Unit quirk that needs to
be handled. And the kernel recovery cases needs to be tested.

Thanks again!

-Yazen