HWPOISON [LWN.net]

HWPOISON

Posted Sep 1, 2009 21:28 UTC (Tue) by dlang (guest, #313)
In reply to: HWPOISON by jzbiciak
Parent article: HWPOISON

ok, I think the question comes down to this

is HWPOISON a hardware level feature or a OS level feature?

if it's a hardware level feature (which is what I understood from the original article) then it wouldn't necessarily cause a machine check error ever.

if this is instead a difference in how the OS responds to a memory error I just completely misunderstood what's happening.

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HWPOISON

Posted Sep 1, 2009 23:59 UTC (Tue) by jzbiciak (guest, #5246) [Link]

The overall "HWPOISON feature" is both a hardware feature and a software feature. There is a hardware component (the newly improved Machine Check Architecture in the CPU), and then there's the OS handler that makes use of it.

A machine check error (whether delivered as an exception or an interrupt--the new MCA does both depending on the error type) is a message from the hardware to the software. In the most recent Intel architectures, they support a notion of "recoverable machine check," wherein the hardware tells the OS that no CPU state was corrupted when it noticed the problem. If you look at that PDF I linked, there are a number of status bits (including AR--Action Required) that indicate the severity of the error. There's a separate table in Intel's PDF that suggests the possible OS responses to a particular error.

Once the hardware delivers the message to the OS (via a machine check), the OS is then free to deal with the machine check however it pleases. For "Action Optional" machine checks that can happen asynchronously to program execution (such as due to scrubbing), the OS can queue up a handler to go deal with the affected page, either by poisoning it or unmapping it or what-have-you. That's the stuff Andi Kleen and co.'s patch does.